Kelly Sotherton climbed into the bronze medal position after four events in the heptathlon at the World Championships.
Sotherton set personal bests in the hurdles and 200m
British team-mate Jessica Ennis slid back to fourth after a poor shot put while defending champion Carolina Kluft holds a 148-point lead over the field.
After a brave race, Devon's Jo Pavey agonisingly came fourth in the 10,000m, won by Ethiopia's Tirunesh Dibaba.
The first title in Japan was won by Kenya's Luke Kibet in the marathon while Reese Hoffa took shot put gold.
It was an encouraging start to the championships for Britain's heptathletes as Ennis and team-mate Sotherton both recorded lifetime bests in the opening 100m hurdles.
Ennis sliced 0.07 seconds off her personal best while Sotherton, 30, achieved her fastest-ever mark of 13.21.
In her favourite event, the high jump, Ennis, 21, struggled in the hot conditions and went over at 1.89m while Sotherton managed 1.86m.
I've got a very good chance of coming out with a medal
After the morning session's two events, Ennis was second behind Kluft, who showed her class with a personal best of 13.15 in the hurdles and 1.95m in the high jump.
But Ennis was undone by the shot put when the action resumed in the evening, as she struggled to 11.93m and slid back to sixth, while Sotherton improved with a season's best of 14.14m.
Ennis came back strongly to move up to fourth by winning the 200m in a lifetime best of 23.15 seconds while Sotherton also bookended her day with a PB of 23.40.
"I started the day with a personal best and ended with one so I'm happy, I've been consistent and that's what I wanted," Sotherton told BBC Sport.
"If I can keep that consistency on Sunday I've got a very good chance of coming out with a medal."
Ennis added: "I had a disaster with the shot put. I knew I had to run a PB in the 200m and that's what I did so I'm made up."
Kluft, who is chasing a third straight world title, leads from Ukraine's Lyudmila Blonska with Sotherton 25 points further back.
Medals are still a real possibility for the British duo with the long jump, the javelin - Sotherton's Achilles heel - and 800m still to come.
Pavey lies on the track after finishing fourth in the 10,000m in humid conditions
Jo Pavey finished outside of the medals once again as she was unable to hang on to third in the 10,000m.
The Devon runner, who was fifth in the 5,000m at the 2004 Olympics, moved into third on the last lap but was overhauled round the final bend by American Kara Goucher and had to settle for fourth.
"I just couldn't hold on," said Pavey, who was making her major championship debut over 10k.
"It's disappointing not to get a medal but it's better than it being a total disaster. I've got to be pleased in some ways but it is so frustrating."
Ethiopia's Tirunesh Dibaba became the first woman to defend a 10,000m title, despite dropping to the back of the field because of stomach pains.
Dibaba won in 31 minutes and 55.41 seconds ahead of Turkey's Elvan Abeylegesse and Goucher.
It's the fastest track I've ever been on
"That was the hardest race of my life," Dibaba said. "I was struggling but I told myself to hang in."
Britain's trio of sprinters, Marlon Devonish, Mark Lewis-Francis and Craig Pickering, sailed into Sunday's 100m semi-finals.
Devonish followed world number one Tyson Gay home in the second quarter-final in 10.13 while Lewis-Francis (10.17) and Pickering (10.21) also reached the last 16.
"I'm in the shape of my life and I'm a championship performer. I plan to get into the final and go hell for leather," said Devonish.
The new track at the Nagai stadium got the approval of Gay, who won his quarter in 10.06.
The American, who owns the fastest time of 9.84 seconds this season, said: "It's the fastest track I've ever been on."
But it was his rival, Jamaica's world record holder Asafa Powell, who powered into the semis as the fastest qualifier in 10.01 seconds.
"It was as I expected, nothing else. All is going according to my plans," Powell said.
But there was an upset in the earlier round as Portugal's Olympic silver medallist and European champion Francis Obikwelu was disqualified for a false start.
Triple-jumper Phillips Idowu made a solid return from a two-month lay-off to ease into Monday's final with a solitary jump of 17.07m.
Idowu is one of Britain's best hopes of an individual medal in Japan
The 28-year-old, who has posted a season's best of 17.35m, has been suffering with a back injury.
"Qualifying is the hardest and you have to make sure you're there in the final," said the Belgrave Harrier. "I put it out there and sat it out.
"I didn't open out in my phases so it looks like there is a lot more there. I feel like I've taken a little holiday but it's nice to be back on an occasion like this. Right now, I still believe I'm the underdog."
Luke Kibet, a 24-year-old prison guard, claimed Kenya's first world marathon gold in 20 years - the first medal of the championships in Osaka.
In stifling heat, he won in two hours 15 minutes 59 seconds, with Qatar's Mubarak Hassan Shami over a minute behind with Swiss Viktor Roethlin third.
Former wrestler Reese Hoffa easily dethroned fellow American Adam Nelson in the men's shot put, with Andrei Mikhenevich of Belarus third.
"We wanted 1-2 and we accomplished that. We had a lot of fun out there and we both executed well," said Hoffa, who landed 22.04m.
Andy Baddeley eased into the 1500m semi-final on Monday, finishing fifth in his heat in 3:39.60.
But Hatti Dean and Helen Clitheroe both failed to win a place in Monday's 3,000m women's steeplechase final, but Ireland's Roisin McGettigan and Fionnula Britton advanced.
Guernsey's Dale Garland also failed to advance in the men's 400m hurdles, finishing last in his heat in 49.98.