Roger Federer suffered a shock straight-sets defeat to Kei Nishikori at the ATP Finals on Sunday, severely denting his bid for the 100th title of his illustrious career.
The Swiss, who has won the season-ending event a record six times, produced an uncharacteristically error-prone and fractious display as the Japanese seventh seed prevailed 7-6 (7/4), 6-3.
The result means Federer is now in danger of failing to qualify for the semi-finals for just the second time in his career.
Federer, 37, has beaten Nishikori in Paris and Shanghai in recent weeks but despite having the backing of a full house at the O2 Arena, he never really settled into a groove.
The normally cool Swiss was warned by the umpire for ball abuse in the 12th game after Nishikori produced a staggering backhand winner down the line.
The Swiss great made 20 unforced errors in the first set and Nishikori capitalised, forcing a tie-break in which he raced to a 6/1 lead before sealing it 7/4.
Federer, showing real urgency, broke Nishikori immediately at the start of the second set but it proved a false dawn as he lost his own serve as his frustrations mounted.
Nishikori broke again in the sixth game and served out for victory.
- Anderson win -
The title is contested by the eight players who have accumulated the most ranking points over the season and is in a round-robin format, with the best four players reaching the knockout semi-finals stage.
Earlier, Kevin Anderson made an impressive ATP Finals debut in the same Lleyton Hewitt group, beating Dominic Thiem 6-3, 7-6 (12/10).
South African giant Anderson came into the match with a healthy 6-2 lead over Thiem in head-to-heads but had lost their two most recent meetings, including a straight-sets defeat at the US Open.
The Wimbledon finalist, who stands at 2.03 metres (six feet eight inches) tall, established an early grip, dominating the early exchanges and securing the crucial break in the fourth game.
The 32-year-old, making his debut at the season finale, won 84 percent of his points on serve in the first set, compared with just 58 percent for his ragged Austrian opponent.
Thiem tightened up his game in the second set and looked far more aggressive, with his serving numbers rocketing higher but Anderson held firm to take the set into a tie-break.
In a see-saw tie-break, 25-year-old Thiem had two chances to level the match but Anderson kept his cool to seal the victory with an ace on his fourth match point.
"I definitely felt a little bit nervous," said Anderson. "But I was able to settle very quickly and find a really good rhythm, taking care of my serve games nicely, created quite a few opportunities on his serve.
"He wasn't serving at, you know, well, a very high first serve percentage... I think overall just getting off to a good start helped me a lot today."
World number one Novak Djokovic, who heads the Guga Kuerten group, will be in action on Monday against John Isner while Alexander Zverev takes on Marin Cilic.
Djokovic, who replaced Rafael Nadal at the top of the rankings last week, was Sunday presented with the 2018 year-end ATP World Tour number one trophy.