Kei Nishikori upsets Roger Federer in ATP Finals opener

Seventh seed Kei Nishikori defeated six-time former champion Roger Federer in straight sets on day one of the prestigious Nitto Association of Tennis Professionals (ATP) Finals here at the O2 Arena on Sunday.

The Japanese ousted Federer, who was attempting to win the 100th title of his career, 7-6(4), 6-3 in a thrilling opening fixture that lasted one hour and 28 minutes.

The two-time semi-finalist of the season finale, Nishikori, said, "I'm glad to win, it is never easy to play with my idol, it is always a big challenge against him, so it was great to win," according to the Nitto ATP Finals.

After going down 5-6 in the first set, Nishikori made a scintillating comeback as he attacked Federer at the backhand to level the game. The Japanese then went onto secure the first game in a tie-break 7-6(4).

As the game advanced to the second match, Nishikori broke Federer's serve for a 4-2 advantage. Federer then made a flurry of errors that allowed Nishikori to end the game in his favour 6-3 and sweep the opening fixture.

The Swiss player, who has dropped to a 46-9 match record in the season, said, "I felt we both struggled throughout the first set."

"I had my chances may be a bit more than he did. Then I started to feel better in the second set. I think we both did. The level went up. Unfortunately, I couldn't keep the lead that I got early. That was important, I think, at the end. That was the key to the match, that sort of - I guess - [a] 10-minute swing at the end of the first throughout maybe 1-1 in the second," Federer said.

Federer will next face sixth-seeded Austrian Dominic Thiem, who lost to the first-time qualifier and fourth seed Kevin Anderson on Tuesday in the round-robin group.