Novak Djokovic secured his place in the last four of the ATP World Tour Finals with a hard-fought victory over Juan Martin del Potro.
The Serbian did not have it all his own way at the season-ending hard-court event in London but eventually prevailed 6-3, 3-6, 6-3.
World number two Djokovic faced two break points in his opening service game but held on before forcing the break himself to go 4-2 up.
Having served out for a 1-0 lead in sets, the second set was a mirror image, the Argentinian breaking to move into a 4-2 lead and soon levelling the match.
Remarkably, the third set continued the pattern, Djokovic winning on Del Potro's serve in the sixth game and tying up the decider 6-3 to progress to the semifinals.
Roger Federer earned his first win at this year's event on by beating Richard Gasquet in straight sets and will now face a shootout with Del Potro.
The Swiss, a six-time winner of the event, was beaten by Djokovic in his opening Group B on Tuesday, but he responded impressively against Frenchman Gasquet.
Sixth seed Federer won 6-4, 6-3 in an hour and 21 minutes to keep his hopes of reaching the semifinals alive.
Federer has never lost a hard-court game against Gasquet and he secured the first break in the third game of the match when the eighth seed hit a simple backhand into the net.
Gasquet responded and took advantage of a double fault and two unforced errors from the Swiss to break back at 4-4.
However, Federer immediately secured his second break when a lengthy rally ended with Gasquet going long with a forehand and the 17-time grand slam winner served out the set.
Federer carried on where he left off by breaking in the first game of the second set and Gasquet failed to convert two break points in the fourth game as the Swiss battled to hold serve.
Gasquet also had an opportunity to break in each of Federer's next two service games, but was unable to peg his opponent back.
The former world number one made hard work of securing victory, with some sloppy errors ensuring he needed a sixth match point to see off Gasquet when the Frenchman netted a forehand.