Novak Djokovic was down early, but not out.
The world's top-ranked player fought back from a set and a break down to earn a 4-6, 6-3, 6-2 win over Juan Martin del Potro in the semifinals of the ATP World Tour Finals.
Djokovic, who was 3-0 in round-robin play during the week, will play for the title on Monday against either Roger Federer or Andy Murray.
Del Potro picked up the first break of serve on Sunday for a 5-4 lead and held at love to close out the first set. He then broke for a 2-1 lead in the second, ending a rally of more than 20 shots with a blistering forehand down the line that Djokovic could not return.
Djokovic quickly broke back, finally converting on his fourth chance of the game and sixth of the match. The tide then appeared to turn, as he broke again to force a third set.
It was all Djokovic in the third. He broke for a 2-1 lead and broke again for 5-2 before holding at love to win it. The Serbian star won all but two of his service points in the dominating third set, capturing 27 of the 40 total points played.
Djokovic won this title in 2008, while Federer is the two-time defending champ and a six-time winner of the season-ending event.