American Jeremy Wariner wraps up a second straight world 400m title in a personal best time of 43.35 seconds - but is short of the world record.
There were also glorious wins for Xiang Liu in the 110m hurdles and Allyson Felix in the women's 200m.
British quartet Christian Malcolm, Craig Pickering, Marlon Devonish and Mark Lewis-Francis qualified for Saturday's 4x100m relay final.
Jamaican Maurice Smith took the lead in the decathlon after five events.
ACTION AS IT HAPPENED (ALL TIMES BST):
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1440: Let's call it a day. It could be another special day for Tyson Gay tomorrow as he targets a third gold medal in the 4x100m relay, but can Britain pull anything out of the bag? Jo Pavey runs her second final of the championships in the 5,000m. Meet you back here, same time on Saturday.
1435: The live action is done for another day in Osaka, but just time to reflect on the 400m. Here's a word from two-time world 400m champion Jeremy Wariner:
"I enjoyed the race today. It felt good and I was able to come through to victory. I knew Lashawn would be there but I also knew if I executed like I needed to, I was going to win."
And here's a word from his mentor Michael Johnson, who will get on the phone to the Texan no doubt:
"I loved the gold shoes. It was a very good run, it was very evenly paced. He was patient. He was able to hold his own and the race was really going to be won really pushing from 200-300m and he did that. I'm really proud of Jeremy's ability to handle the pressure, but the important thing was to win another world championship, which only I have done before."
1422: Xiang comes through strongly to pop over the 10 hurdles in 12.95. American Terence Trammell gave him a good race up until three hurdles to go but the Chinese athlete pulled away. David Payne took third in a personal best of 13.02, despite only being handed a late call-up to the team.
"There is pressure from around the world and back home in China and that will be a relief for Xiang. Xiang had a good start but it was only after the last couple of flights that he got it. He looked across, once, twice its mine and now looking forward to an incredible Olympic Games in Beijing next year."
BBC commentator Steve Cram
1419: There is just the 110m hurdles final to come where Olympic champion and world record holder Liu Xiang is chasing his first world title. Can he do it?
1415: Jamaica's Maurice Smith leads the decathlon at the halfway point. He clocked 47.48 in day one's closing 400m. Dmitry Karpov moved up to silver with a run of 47.44 - 86 points behind - while Olympic champion Roman Sebrle is in third, just five seconds adrift.
1406: Jeremy Wariner and Lashawn Merritt are toe-to-toe coming off the top bend but Wariner pulls away down the home straight to defend his title in a new personal best of 43.45. It is an American one-two-three as predicted as Merritt (43.96) takes second ahead of Angelo Taylor (44.32).
1403: There is apparently a strong breeze down the home straight, which could affect the times in Japan ahead of the 400m final.
1400: Bryan Clay's defence of his decathlon title is over, as he has pulled out of the competition. The American hurt his right leg when attempting the high jump at 2.00m. If you're eating lunch look away now. "He heard something pop," said his agent. Sorry, makes me squeamish anyway.
1359: Czech Barbora Spotakova claims the javelin crown after launching 67.07m in the third round. World number one Christina Obergfoll takes silver (66.46) ahead of German compatriot Steffi Nerius (64.42).
1345: In the second heat of the decathlon 400m, Sebrle comes home third in a season's best of 48.80. Still waiting to see if Clay turns up in the third heat. Sebrle is in second at the minute but we'll update you on the full standings when the heats have finished. The muscle-bound Czech tells BBC Sport he is still struggling to acclimatise:
"I still feel very bad in Japan because of the changing times but I will fight. I miss the world title and I want it."
1332: Bad news for defending decathlon champion Bryan Clay. The American did injure himself while attempting a high jump earlier in the evening session and he is reportedly close to pulling out of the remaining six events. He was lying in third too.
1320: What a run. American Allyson Felix tears away from Veronica Campbell and Torri Edwards coming out of the bend and stretches away to win the 200m by half a second clear in a personal best 21.81 seconds. Felix defends her title at the age of 21. Campbell, the 100m champion, took second in a season's best 22.34 while Sri Lanka's Susanthika Jayasinghe snuck through for bronze.
"That's such a phenomenal time at such a young age. She generates such a lot of power and speed. Her speed endurance has taken her to another level."
BBC pundit and five-time Olympic champion Michael Johnson
1312: Women's 200m next up on the track, like really soon. Veronica Campbell is going for the sprint double after winning the 100m crown on Monday. False start - good I can eat a bit more of my pitta.
1306: "It was a good time, safe changeovers, but none of the British four looked particularly sharp in the 4x100m. I hope that changes tomorrow!"
Isabre on 606
1257: We are getting a bit behind with the field events, so let's rectify that. Cuba's Yargelis Savigne came out big with her first triple jump, landing 15.28m and no one could match that. Long jump champion Tatyana Lebedeva (15.07m) claimed silver with Greece's Hrysopiyi Devetzi (15.04m) third.
The women's javelin final is also up and running, and Czech Barbora Spotakova has taken a big lead, landing 66.40m.
1248: The world record holder Asafa Powell leads home the Jamaican quartet in the second relay heat. He finished like a train. Now the Jamaicans have a world-leading time of 38.10. No Tyson Gay for the Americans today but still they come through in second while the crowd get to their feet as Japan qualify in third.
1242: Phew! Christian Malcolm, Craig Pickering, Marlon Devonish and Mark Lewis-Francis are safely into Saturday's final, but there might be a little question mark over their speed if they are to challenge the Americans for gold. Still they go through in 38.33 behind Brazil, who set a world-leading time of 38.27. Poland come third.
1240: Nerves, jitters, nail-biting - Britain's 4x100m squad are about to try and qualify for the final. Christian Malcolm to lead off.
1236: Just want to catch up with Rimmer. He came home in 1:47.39 in sixth behind winner, Olympic champion, Yuriy Borzakovskiy. He tells BBC Sport:
"I wanted to go out super quick and it turned out that I was in the lead. But I was conscious that they were behind me. As soon as I hit the front I was running scared and these guys can smell that on you. I had half an eye behind me and you can't do that. I should have concentrated on my own race and I've learned a lot from that."
1230: The decathlon high jump competition is over and there is a change in the top three as Roman Sebrle forces himself into the silver medal position. The Czech soars over at a season's best of 2.12m and that is good enough to climb just 28 points behind Jamaican leader Maurice Smith while defending champion Bryan Clay drops to third, seven points behind Sebrle.
1222: Rimmer takes a massive lead round the back straight but can he keep it going all the way to the line? The answer is no. Sadly he has nothing left down the home straight and is reeled in by the pack. He trails home in sixth. Arrow down. Oh.
"One thing you can be sure of he won't do that again. He's a better runner than that. It's difficult to take it away from the a field that contains talent like that. When you find yourself at the front you have to re-think and ask 'am I supposed to be here?' If the answer is no then you should stop - but he didn't."
BBC commentator Steve Cram
1219: Rimmer goes to the front through the first 200m is that the right move, asks Steve Cram?
1218: How will Liverpool's Michael Rimmer do in the next semi. He slaps his face on the start line, so let's see how he goes.
1213: More relentless pace in the next semi, so fast that Brendan Foster can hardly keep up with it all. Canada's Gary Reed comes through strongly to win in 1:44.53, but Rashid Ramzi - the defending champion - is out. He's got a silver 1500m medal to cheer him up though.
1207: The men's 800m semi-finals are under way, but they are very tough and Britain's Michael Rimmer will have to run out of his skin to qualify. Only the first two and then the two fastest losers will go through to Sunday's final. Kenyan's Alfred Kirwa Yego takes a very competitive first heat in 1:44.54.
1205: The women's triple jump final is under way and Cuban Yargelis Savigne has stolen the march on 2001 and 2003 champion Tatyana Lebedeva as she lands a world-leading leap of 15.28m
1200: Wobble, wobble and, er, a wobble but the bar stays on for Sebrle at 2.06m in the decathlon high jump. Bryan Clay and leader Maurice Smith both bowed out at 1.97m. And look at Sebrle go now - he's only gone over at 2.09m.
1155: Bit of a tactical problem for Lisa Dobriskey in the second semi-final. She trips over on the second lap - a bad day for Brit bashing it seems - and then she gets caught in traffic on the inside at the bell. The Commonwealth champion crosses the line out of contention, and the reason becomes clear - she was running with one shoe! She tells BBC Sport:
"I lost my spike with 600m to go because it got caught and then I had to kick it off. It knocked my rhythm a bit, and I'm gutted because I felt strong."
Plucky Brit for carrying on running sans shoe - almost deserves a Brit up arrow, but down it goes all the same.
1140: At the bell, Britain's Abby Westley is at the back, after getting badly bumped in the second lap. She injects some pace to finish strongly, but not enough to get amongst the top five and the 20-year-old's inexperience probably let her down there. Bahrain's Maryam Yusuf Jamal won the heat by a long way in four minutes 14.86 seconds.
1134: Right then, I'll be getting the Brit arrow out shortly as the women's 1500m semi-finals are about to get under way. We have a British representative in each - with the top five qualifying and then the two fastest losers. Let's see how we go on ...
1130: Sebrle could gain some ground on his rivals as he clears 2.03m in the decathlon high jump. The Czech is lying in fourth but could improve at this rate.
1120: There's been a lot of chat already looking ahead to the men's 4x100m relay where - if they get through the heats - Britain have high hopes of improving on their Helsinki bronze. Christian Malcolm was part of the quartet who won that bronze at the last Worlds but Colin Jackson says he is a bit surprised to see him back again.
"Christian has had a lot of problems this year with an Achilles tendon injury and it's painful to shake off, so I'm surprised the selectors are taking the risk on him as he is not at full pelt."
BBC pundit Colin Jackson
Let us know what you think of Britain's relay chances on 606.
1110: By the way, there is some live action at the Nagai stadium as the decathletes are cracking on with the high jump. Leader Maurice Smith goes clear at 1.97m as does Bryan Clay while Olympic champion Roman Sebrle clears at 2.00m
1105: "I think it's pushing it a bit to expect Warriner to win the 400 metres final with a record. Yes, he's the favourite to win it, but don't rule out Angelo Taylor and LaShawn Merritt...."
Mikesiva on 606
1100: Those of us up with the lark have sat surveying the comings and goings of the deserted Nagai stadium in Osaka.
The 1500m wheelchair racers have been practising and that's pretty much it - apart from a bunch of officials in fetching yellow coats who have been lounging about chatting for ages.
Things should hot up later when five gold medals will be decided in the women's triple jump, javelin, 200m and the men's 400m and 110m hurdles.
The pick of the day? It has got to be Jeremy Wariner in the 400m as he sets his sights on gold and the world record.
Keep those fingers crossed for the British men in the 4x100m relay. They couldn't get the baton round safely at the London Grand Prix and we don't want to see more shaky hands in Japan.