Following a straight-sets upset over six-time champion Roger Federer in his opening match, Kei Nishikori came crashing back to earth with a 6-0, 6-1 loss to South Africa's Kevin Anderson at the ATP Finals on Tuesday.

After beating Anderson earlier this month at the ATP Masters in Paris, Nishikori was outgunned by the South African in just 1 hour, 4 minutes at London's O2 Arena.

The heavy loss came less than 48 hours after world No. 9 Nishikori handed 20-time Grand Slam champion Federer his first straight-sets loss in round-robin play at the tournament.

Narrowly avoiding just the second 6-0, 6-0 scoreline in tournament history, the 28-year-old Nishikori fell to his third loss in four meetings this year against the 32-year-old Anderson, ranked No. 6.

The 2.03-meter Anderson served 10 aces, including three in a row in the third game of the opening set.

"He was hitting the ball really well and it was making me impatient. I wasn't able to extend out rallies," Nishikori said.

"My shots just weren't going in. I was out of rhythm and couldn't find it in time."

Nishikori, whose strong run of form in the second half of 2018 included a U.S. Open semifinals berth, will meet world No. 8 Dominic Thiem in his next round-robin match.

He holds a 3-1 career advantage over the Austrian head to head, but the pair have split their two matches this year.

"I'm going to put in a solid practice so I can go into the match positively," Nishikori said.

The Nov. 11-18 tournament, which features the top eight singles players and doubles teams, has a round-robin format. The top two in each group progress to the knockout semifinals.

With two wins from two matches, Anderson leads Group Lleyton Hewitt ahead of Nishikori and Federer on a win and a loss apiece, while Thiem has two defeats.