As soon as it looked like Novak Djokovic was headed for his offseason vacation, the top-ranked Serb started playing his best tennis.
Djokovic was broken late in the first set and again early in the second, but he then found his stroke and quickly rebounded to beat Juan Martin del Potro 4-6, 6-3, 6-2 Sunday and reach the championship match at the ATP finals.
"He was set and a break up. He was the better player on the court throughout the first part of the match," Djokovic said. "I managed to turn it around and played better."
Djokovic will face either six-time champion Roger Federer or Andy Murray in Monday's final. Federer and Murray were to play later Sunday in the other semifinal match.
Del Potro took the lead late in the first set, breaking Djokovic for a 5-4 lead before serving out the set at love.
The tall Argentine, who defeated Djokovic to win the Olympic bronze medal at Wimbledon this year, then went up a break early in the second set. But Djokovic broke right back to 2-2 and then took Del Potro's serve again to take a 5-3 lead.
In the third, Djokovic decided things by winning 12 of 13 points in a three-game stretch to take a 3-1 lead. He broke again three games later and held at love to advance.
The victory was Djokovic's 74th of the season, tying him with David Ferrer for the most this year. It was also his seventh win in nine matches against Del Potro.
Djokovic won the ATP finals in 2008, and reached the semifinals in 2009 — the first year the tournament was played at the O2 Arena in London.
In the other semifinal match, it will be the third meeting in London this year between Federer and Murray. Federer beat Murray in the Wimbledon final to win his seventh title at the All England Club, but Murray bounced back to beat Federer in the Olympic final on the same Centre Court grass.