Andy Murray will lead the Great Britain team in next week's Davis Cup final against Belgium in Ghent
Andy Murray missed out on a place in the last four at the ATP World Tour Finals as Stan Wawrinka won their final group match in straight sets.
The Swiss fourth seed came through 7-6 (7-4) 6-4 in London and will play Roger Federer in Saturday's semi-finals.
World number one Novak Djokovic will take on fifth seed Rafael Nadal in the other half of the draw.
Murray heads to Belgium next week for Great Britain's first Davis Cup final since 1978.
"I didn't find it difficult not thinking about the Davis Cup Final, to be honest," said Murray.
"Obviously the only positive for me this week is I've come away from it injury-free. Now I have a couple more days to get ready for Belgium."
With the remaining semi-final place coming down to a straight contest between Murray and Wawrinka, the 17,000 fans at the O2 Arena settled in for something close to a Grand Slam quarter-final.
Over the course of one hour and 53 minutes they were treated to some great shot-making, unexpected misses, a late Wawrinka collapse and a furious racquet-smashing Murray.
The home favourite ended the match with 22 winners but 30 unforced errors, as his ATP season drew to a close with back-to-back defeats.
"My timing wasn't there certainly the last few days," said Murray.
"I made way too many errors the last two matches. Everyone obviously can make mistakes, it's just more on easy shots, cheap errors.
"That's something for the most part of this year I haven't been doing."
Any thoughts of a Davis Cup distraction appeared misguided when the Briton roared in delight after a brilliant early forehand pass on the run, and he hit back from dropping serve in game eight with a fabulous winner off a Wawrinka smash.
Murray looked well set at 4-2 up in the tie-break, but his radar then seriously malfunctioned and two errant forehands contributed as Wawrinka reeled off five straight points.
An agitated Murray spent the changeover deep in discussion with an official and proceeded to drop his opening service game of the second set.
The match only went in one direction for the next half-hour, with Wawrinka racking up the winners and Murray the errors, before it was the French Open champion's turn to look fragile at 5-2.
Andy Murray returning to court after his defeat to work on his game in front of a deserted O2 Arena
His first serve disappeared and Murray made one last push, trying to get the crowd involved as he cut the deficit before smashing his racquet when a chance to level slipped past.
A 15th forehand error of the night finally brought an end to Murray's challenge, and took Wawrinka through to the semis for the third year in a row.
Such was Murray's disappointment with his timing, he headed back out onto court after his media duties at 11pm to hit for a few minutes more with coach Jonas Bjorkman.
Great Britain coach Leon Smith (far left) was among those in Murray's unusually distant box
Murray's support staff and family were absent from their usual courtside position, instead watching the action from up in the stands.
In their place, arena staff members occupied two seats in an otherwise empty box.
"With my team sitting behind, I just felt like sometimes when the box is extremely close to the court, I sometimes can find that a distraction," Murray explained.
"So I thought it would be better to have them sit a bit further away from the court. That was it."
Nadal warmed up for his semi-final against Djokovic with a 6-7 (2-7) 6-3 6-4 win over fellow Spaniard David Ferrer in his final round-robin match.
The world number four spent two hours 37 minutes on court and has a short turnaround before returning to play the Serb at 14:00 GMT on Saturday.
Ferrer had already been eliminated after two defeats at the O2 Arena.
It was the first time in 44 matches he has lost after wining the first set.
"Novak is playing almost better than impossible. He is achieving almost everything that one player can dream. I have to play to the limit of my best to have a chance," said Nadal, 29, in his on-court interview.
Nadal's meeting with Ferrer may have been a dead rubber in tournament terms but the lure of £109,000 - the prize money on offer for every victory in the round-robin stage - meant that there was at least a financial incentive.
Ferrer won a curiously low-key and inconsistent first set on a tie-break after each player had broken the other's serve on three occasions.
But Nadal, who was down at 10th in the world in August after a poor start to the year, edged the second with a solitary break before accelerating away at the end of the third.
Nadal will take to the court with a 23-22 lead over Novak Djokovic from their previous meetings