Roger Federer's candidacy to win his 100th title in the ATP Finals hit the buffers in his first match against Kei Nishikori.
The Swiss star of 37, trying to succeed at O2 Arena for the seventh time and first since 2011, was defeated in direct sets by the Japanese world number nine.
Federer won his 15 opening matches at the season-ending event, but placed an error display in his first round robin game in the London 02.
Nishikori lost his previous six games against Federer but, after winning a first tie, he kept his nerve to surprise, but he deserved 7-6 (4) 6-4 wins.
"I'm glad to win, it's never easy to play with my idol," said Nishikori.
"It's always a great challenge to play Roger. The first game is not easy, we both played a bit, but the second set was better.
"I lost twice in the last two months, then I played more aggressively and things started to work, especially in the second set. There were some points of luck but I played well today."
Federer made 36 unforced errors without specifying, 20 in the first set, but that did not take anything away from Nishikori.
After a scandalous downhill volley on the net to help bring the first set to a draw, Nishikori made the fourth of the five points ahead.
Federer seemed to mean business when he broke Nishikori in the first game of the second set, but later handed over the initiative back with a desperate service game.
In his place, it was Nishikori who broke again to lead 4-2 and – after Federer forced him to serve the party – the 28-year-old watchman forced a memorable victory.
Federer admitted that his practice was "a bit around the place" but he had no excuses.
"I felt we both strived for the first set. I could say it was a first lap," he said.
"I had my possibilities maybe a little more than what he did. So I started feeling better in the second set. I think we both did. The level went up but unfortunately I was not able to keep the lead I've had."
Federer, of course, still has the opportunity to move forward in the tournament.
However, he will be watching for a great obstacle in his group in the form of Kevin Anderson, 6 foot 8 in South Africa who knocked him down this year at Wimbledon.
Anderson, who came to the final in the SW19 – through his epic six-hour semifinal victory over John Isner – is making his first appearance at ATP Finals at 32.
He seemed determined to compensate for the time lost when defeating the Austrian Dominic Thiem in direct sets, 6-3 6-6 (10)
The French finalist Open Thiem simply could not penetrate in the Anderson service and admitted that he lost before a potential title.
The 25-year-old said: "It's very dangerous. It's a good thing. I think that, on the part of the tallest youth, you're playing from the baseline. Of course, you can win the title here."
The number one in the world Novak Djokovic opens his campaign against Isner on Monday night.
Briton Jamie Murray and Brazilian partner Bruno Soares won their double role of opening role, overtaking Raven Klaasen from South Africa and Michael Venus from New Zealand. 7-6 (5) 4-6 10-5.
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