Juan Martin del Potro held on to beat Fernando Verdasco 6-4, 3-6, 7-6 (1) Tuesday in his second round-robin match of the ATP World Tour Finals.
The fifth-ranked Argentine wasted two match points in the third set and was broken while serving for the match before finally closing it out.
In the first set, del Potro dominated on his serve, losing only three points until the final game. In the second, Verdasco broke for 5-3 before evening the match on his serve.
Del Potro lost to Andy Murray in his opening Group A match. He most likely will have to beat four-time champion Roger Federer on Thursday to reach the semifinals of the season-ending tournament for the top eight players. Verdasco, with two losses, will face Murray in his next match at O2 Arena.
Federer played Murray later in the day in the other Group A match.
On Wednesday, Andy Ram and Max Mirnyi (7) face Lukasz Kubot and Oliver Marach (6) in their second match of the round robin stage. The Israeli/Belarusian defeated Bob and Mike Bryan (2) in straight sets in their first match on Monday.
In Group B, Novak Djokovic and Robin Soderling won opening matches and will play each other Wednesday. Rafael Nadal, who lost to Soderling on Monday, will face Nikolay Davydenko.
Djokovic rallied from a poor start to beat Nikolay Davydenko 3-6, 6-4, 7-5 in the late match Monday and moved a step closer to the semifinals of the season-ending tournament for the top eight players in the world.
Djokovic entered the tournament on a hot streak after winning his last two tournaments but struggled early against Davydenko, losing his serve twice in the first set.
He grew increasingly frustrated in the second, throwing his racket in disgust after netting an easy shot that would have set up a break point in the third game.
"I wasn't feeling that great on the court," Djokovic said. "Physically-wise, I was a bit slow, kind of couldn't get in great rhythm."
AP contributed to this report