ATP World Tour Finals 2018 Results: Sunday Tennis Scores and Updated Schedule

Roger Federer slipped to a surprise straight-sets defeat against Kei Nishikori at the 2018 ATP World Tour finals on Sunday.

Earlier, Kevin Anderson drew first blood in the round-robin when he beat Dominic Thiem in the opening match of the tournament at the 02 Arena in in London.

The pair opened proceedings in Group Lleyton Hewitt, and Anderson got off to the best possible start with a victory in straight sets.

Sunday's Results, Group Lleyton Hewitt

(4) Kevin Anderson bt. (6) Dominic Thiem: 6-2, 7-6 (10)

(7) Kei Nishikori bt. (2) Roger Federer: 7-6(4), 6-3

Monday's Schedule, Group Guga Kuerten

Not before 2 p.m. GMT/10 a.m. ET: (3) Alexander Zverev vs. (5) Marin Cilic

Not before 8 p.m. GMT/3 p.m. ET: (1) Novak Djokovic vs. (8) John Isner

Federer's struggles were evident early on, despite Nihiskori needing to win a tiebreak before salting away the opening set on Centre Court.

Things were harder still for Federer in the second, as Nishikori attacked the 37-year-old's serve. Federer earned a break of his own to gain some hope:

However, his dangerous opponent kept on breaking back:

Nishikori's latest break left Federer on the brink. He couldn't cope with Nishikori's range of shots, while he also failed to deal with a fairly routine serve as Federer didn't break once during the first set.

Some of Federer's key shots let him down, with his forehand leading to more than a few unforced errors, including for the decisive point.

It all added up to an unwanted piece of history for the 20-time Grand Slam champion:

In the day's opening fixture, Anderson took on Thiem, and it quickly became apparent the South African was in fine fettle:

Soon after, he claimed the decisive break in the first set after a double fault and an unforced error from Thiem handed him two break points.

A hold to love consolidated his lead, and he had served out the opener in 41 minutes.

Tennis commentator and writer Jose Morgado was impressed:

The second set was a tighter affair, with Thiem holding his first four service games to love to keep Anderson at bay.

His opponent was similarly resolute, though, and the pair inevitably reached a tiebreaker.

In a suitably close tiebreaker, Anderson came out on top after Thiem had saved three match points.