Novak Djokovic saved a match point in the third set against Tomas Berdych before winning 3-6, 6-3, 7-6 (3) on Monday to avoid becoming the second upset of the day at the ATP World Tour Finals.
Third-ranked Andy Murray struggled through an earlier 6-4, 7-5 loss to David Ferrer with a groin injury he fears could end his tournament. Murray said he would decide Tuesday whether he will pull out.
"If it wasn't Slams or this event, I wouldn't have played," Murray said.
Djokovic, who has been nursing a shoulder problem, withstood some spectacular hitting by the seventh-seeded Berdych, who had a point to win the match at 6-5 in the decider but put a forehand into the net.
Berdych collapsed in the tiebreaker and a relieved Djokovic took his 2011 win-loss record to 70-4.
"I wasn't very satisfied with my performance tonight," Djokovic said. "I know I wasn't playing on top of my game."
The day's events appeared to provide visible evidence of why the top players want the season to be shortened.
Djokovic's stellar season has been blighted by injury of late, and the Serb made a slow start against Berdych before winning in 2 hours, 38 minutes. The winner of three Grand Slams this year just about remained on track for an 11th title of the year.
"I haven't been playing my best for maybe a month and a half," Djokovic said. "I didn't get a lot of indoor match play. And it's been a long season."
Murray was pessimistic about his chances of playing on but said it would be "really gutting" to have to withdraw. If he decides to continue, he will face Berdych on Wednesday, otherwise reserve player Janko Tipsarevic of Serbia will replace him.
"It's been a long season for all of us," Djokovic said. "It's probably taken its toll, maybe for him (Murray) at the wrong moment."
Roger Federer and Rafael Nadal, the other members of the top four alongside Djokovic and Murray, have both recently taken breaks to rest their aching bodies.
Nadal struggled with sickness in his opening win over Mardy Fish but came through in three sets, while defending champion Federer also went the distance against Jo-Wilfried Tsonga.
Nadal and Federer will meet on Tuesday for the 26th time. Nadal leads the head-to-head 17-8.
It was Murray who raised the possibility of a strike earlier in the season in protest at the hectic schedule, while Nadal and Andy Roddick were also vocal in calling for change.
The strike threat receded as the top players struggled to organize a meeting to formulate the plan, but with two of the top four pacquisitionslayers as well as Fish (hamstring) battling injuries at the season's end, the concerns remain.
Djokovic's struggles, however, were as much due to Berdych's bold hitting as his own recent lack of match play.
The 26-year-old Czech had beaten Djokovic once in eight attempts but his forehand began finding its mark immediately and he quickly established a 4-0 lead.
His near-perfect form inevitably faded as Djokovic's improved but he held his nerve and powered a backhand down the line to take the first set.
As Djokovic began to serve with more conviction, Berdych faltered. After missing the match point in the third set, he seemed to lose all focus and one wild forehand in the tiebreaker when the court was gaping drew gasps of shock from the crowd.
"Maybe I was too risky, but that's how I play," Berdych said.
While Berdych played well before fading at the end, Ferrer said his form was on par with with his two recent straight-set losses to Murray at Tokyo and Shanghai.
"I played very consistent all the match," Ferrer said. "Maybe the first set I play better than the second. In the second sometimes I was a little bit nervous. In important moments, I take my chance, and nothing else."
British boxer David Haye was in the crowd and tweeted that he was looking forward to seeing his friend Murray "smash the Spaniard!" Instead, Murray repeatedly swung and missed.
Ferrer broke in the 10th game of the first set and twice came from a break down in the second to stun Murray and the home crowd inside London's O2 Arena.
Murray finished with 44 unforced errors and made just 46 percent of his first serves.
"You've got to sometimes do the right thing," Murray said. "I played, didn't feel particularly great. I'll see if I feel any better tomorrow."