Roger Federer has suffered his first ever straight sets defeat in the group stages of the ATP Finals, losing 7-6 6-3 to seventh seed Kei Nishikori.
The victory marks Nishikori's third career win over Federer in 10 meetings, and had not beaten the 20-time Grand Slam champion since March 2014 in Miami.
Nishikori wrapped up the Group Lleyton Hewitt round-robin match in just 87 minutes.
Kei Upset@keinishikori upsets six-time #NittoATPFinals champion 🏆 @rogerfederer
— ATP World Tour (@ATPWorldTour) November 11, 2018
➡️ https://t.co/hbZ6J6T6lA#ATP pic.twitter.com/yjrUUpKZ5y
Taking his 43rd win of the season, the Japanese returns to the ATP finals for the fourth time in five years.
The two-time semi-finalist failed to qualify last year due to a wrist injury which forced the 28-year-old out of the world's top 20 for the first time since March 2014.
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After sitting at World No. 36 in April, Nishikori climbed his way back up the ATP Rankings with three runner-up finishes at the Rolex Monte-Carlo Masters, the Rakuten Japan Open Tennis Championships 2018 and the Erste Bank Open 500 in Vienna.
Federer, who is a six-time former champion of the ATP Finals, enters the season finale just one victory away from his 100th career title.
The loss to Nishikori marks his first ever round-one defeat at The O2 since 2013, where he lost to Novak Djokovic 6–4, 6–7, 6–2. The Swiss still qualified for the semi-finals, where he lost to top seeded Rafael Nadal 7-5, 6-3.
Federer has won more titles in London than any other city, with eight Wimbledon championships and two (out of his six) ATP Finals triumphs at The O2.
The 37-year-old lifted the Finals Trophy in 2003-04, 2006-07, 2010-11.