ATP World Tour Finals
Venue: O2 Arena, London Date: 22-29 November
Coverage: Live on BBC Two, streamed on BBC website, 29 November, 1400 GMT; 5 live web commentary 1400 GMT; live text commentary on BBC Sport website; live coverage on Sky Sports
The Argentine recovered from a set down to see off Soderling at the O2 Arena
By Piers Newbery
BBC Sport at the O2 Arena
US Open champion Juan Martin del Potro beat Robin Soderling in a heavyweight encounter to reach the final of the season-ending ATP World Tour Finals for the first time.
Fifth seed Del Potro won 6-7 (1-7) 6-3 7-6 (7-3) to set up a decider against Russian sixth seed Nikolay Davydenko, a surprise 6-2 4-6 7-5 winner against top seed Roger Federer.
And after edging the second semi-final, which proved to be the latest in a series of spectacular contests at the O2 Arena, Del Potro is now one victory from landing the biggest title outside the four Grand Slams.
"It was a very, very tough match," said Del Potro. "It was so close, we played unbelievable points.
"The crowd was so excited, cheering for both players, and that helped me to fight. Finally I broke his serve after 4-2 and I started to play a little better, with more confidence. I think that was the key."
Both men came through the round-robin stage with two wins and a defeat but Del Potro arrived in the semis on the back of a win over Federer, while Soderling had already qualified before he lost to Davydenko.
The pair had played twice before, with each man claiming a win apiece, but since their last meeting back in January both have joined the game's elite.
Soderling ended Rafael Nadal's unbeaten run on his way to the French Open final, while Del Potro, 21, landed a first Grand Slam title by ending Federer's dominance at the US Open.
Saturday's clash pitted two of the biggest forehands in the game against each other, as well as two mighty serves, and chances to break were at a premium.
That did not look like being the case when Del Potro earned three points at 0-40 in game two but Soderling saved them all - one with a daring forehand that clipped the line - and the Swede saw off a fourth with an ace in game five.
It was Del Potro who was making the running, and one huge cross-court forehand drew gasps from many of the 17,000 spectators, but his early success with blocking returns did not continue and a tie-break was required.
I don't think I did anything wrong at the end of the match, he just played a little bit better than me
The Argentine made a dreadful start with a double-fault and a forehand that caught the tape and dropped wide, before two more wayward forehands put Soderling 6-1 up and the Swede closed it out with an ace.
Del Potro finally got his forehand going again to smack a return winner for 30-40 in game six of the second set, but Soderling calmly fired an ace down the middle that left the Argentine bent double in frustration.
Many players would have wilted but Del Potro came back stronger, pumping his fist after a lob took him to deuce in game eight and following up with a fierce forehand return as he finally broke serve for the first time.
The momentum carried him to the set and the enthralled crowd roared their approval at the prospect of a decider - which proved to be one of the most dramatic of the week.
Del Potro remained the man most likely, a couple of spectacular backhand winners in game four especially impressive, but none of it rattled Soderling, who struck with two sweetly-timed backhand returns to earn and convert his first break point of the night to make it 4-2.
The shot-making ability of Del Potro meant he was still dangerous though, and two flashing returns saw him recover the break immediately as the dominance of serve gave way, if only momentarily.
Soderling gave it his all but was finally edged out by the Argentine
A final set tie-break seemed the appropriate way to conclude such a tight match, and Del Potro dominated the shoot-out after opening with a raking backhand winner and an ace.
After two hours and 10 minutes the man from Tandil found himself with four match points and he converted the second with an ace before raising his arms in triumph.
"Losing 7-6 in the third in the semi-finals here, it's pretty disappointing," said Soderling. "It was a very tough match.
"Maybe I could have l played a little bit better in the important moments and taken advantage of my chances but, overall, not much I could do. I don't think I did anything wrong at the end of the match, he just played a little bit better than me."
Del Potro has lost two of his three matches against Davydenko, and the Argentine said: "He's very fast and he plays well on this kind of surface, but if I play good, I think I'll have a chance to win.
"I have nothing to lose tomorrow, so I want to enjoy the final and then have a good vacation."
In the evening doubles semi-final, second seeds Bob and Mike Bryan beat third seeds Mahesh Bhupathi and Mark Knowles 6-4 6-4 to set up a final against seventh seeds Max Mirnyi and Andy Ram.