Andy Murray made a disappointing start to his ATP World Tour Finals campaign with a straight-sets defeat by Japan's Kei Nishikori.
The Briton fought back from 4-1 down in the second set to level, but Nishikori broke serve for a fourth time to win 6-4 6-4.
With six-time champion Roger Federer and in-form Milos Raonic still to play, Murray faces a tough task to claim one of the two semi-final places from Group B.
He will meet Raonic in Tuesday's evening session, the Canadian having lost to Federer on Sunday.
After an exhausting six-week schedule that saw him play 23 matches and win three titles to earn his place in London, Murray could not carry that form into his opening match.
World number five Nishikori, 24, overcame a nervous start on his ATP Finals debut and built the pressure on Murray, who made 28 errors and won just 27% of points on his second serve.
"I didn't serve well enough," said Murray. "He was able to dictate a lot of points, especially behind my second serve.
"That was the part of the match where he had the upper hand. That was the difference."
Murray, 27, had been given a warm reception from the O2 Arena crowd on his first appearance in the UK since Wimbledon, and the Scot looked well set after Nishikori double-faulted to drop serve at 3-2.
The match then shifted momentum dramatically and it would ultimately prove decisive in a desperate service game from Murray.
Errors off both backhand and forehand sides left him at 0-40, and a wild double fault gave up the game, allowing Nishikori back into the set.
Murray had his chances early on as Nishikori made just 40% of first serves and threw in five double faults, but the Briton could not find his timing on the return.
He was made to pay when his opponent crunched a second-serve return to earn two set points in game 10, and Murray could only find the net with an attempted drop shot on the second.
Dominating from the baseline and consistently threatening the Murray serve, Nishikori reeled off five games in a row and Murray did well not to fall 4-0 down in the second set as he clung on in a nine-minute service game.
There has been plenty of evidence of Murray's fighting spirit in recent weeks, twice saving five match points to win ATP finals against Tommy Robredo, and another comeback seemed possible when the Scot hauled himself level at 4-4.
A visit from the trainer to give his calf a quick rub might have revealed the strain of his indoor season taking its toll, however, and Nishikori reacted superbly to the mini crisis.
He stopped the rot with a hold of serve to lead 5-4, a brilliant overhead making up for an eighth double fault, and the pressure was back on Murray.
The former Wimbledon champion duly netted a backhand to fall 15-40 and two match points down, and could only fire a backhand long to leave himself facing a battle to qualify.
"It's harder to qualify when you lose your first match, that's pretty obvious," added Murray.
"But unlike the other events, you still have a chance to go through. If this were anywhere else, I would be out of the tournament.
"You need to try to forget about today, work on some things tomorrow, and hopefully play better on Tuesday."