Kei Nishikori ended a six-match losing streak against Roger Federer with a 7-6 (7-4), 6-3 win on the opening day of the ATP Finals in London on Sunday.

Japan’s Nishikori had only beaten the 37-year-old Swiss record-breaker twice previously, with his last success coming in the quarter-finals of the 2014 Miami Masters.

Federer, who had been aiming to claim a 100th career title at the season-ending event, started strongly on serve but his work was undone through a plethora of 34 unforced errors.

"You could tell it was sort of a first round," Federer said.

"I had my chances maybe a bit more than he did. Yeah, then I started to feel better in the second set. I think we both did. The level went up."

His poor run of fortune continued though and with his usual trusted backhand out of tune, Federer conceded two breaks in the second set to lose an ATP Finals opening match for the first time since 2013.

"He deserves credit and respect," Federer said of Nishikori.

"I think we all have that for Kei because he's a great fighter."

The first set was deservedly decided by a tie-break after an intense display of first-attack tennis from both men who have a combined total of 20 appearances at the ATP Finals, with six-time champion Federer beginning his 16th campaign.

The 20-time grand slam champion could have won the first set in game 12 if he had capitalised on the 0-30 pressure he put on Nishikori’s serve, but the world number nine stood firm to force a breaker where Federer unravelled.

The world number three gave away six straight points through errors and even though he fought to save two set points, a seventh mistake into the net secured the lead for Nishikori.

Federer put his first set frustrations to good use in the second set to gain an opening break but he could not cease the opportunity as his 28-year-old opponent hit back with an immediate break to remain on level terms.

And Nishikori finally found the crucial breakthrough he needed to topple Federer as he broke for a 4-2 lead before consolidating his work behind a heavy serve with victory now in sight.

On his next service game, Nishikori rallied to win in one hour and 27 minutes, which ended with one more Federer misfire.

Earlier, South Africa’s Kevin Anderson opened the eight-man tournament with a 6-3, 7-6 (12-10) success over Austrian Dominic Thiem at the O2 Arena.

This year’s Wimbledon finalist Anderson was making his ATP Finals debut at the age of 32 against another grand slam runner-up Thiem, who was bested by Rafael Nadal at the French Open.

"I definitely felt a little bit nervous," Anderson said.

"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."

A netted return from Thiem opened two break point chances for Anderson, and even though he saved one with a lashed forehand, his next attempt landed long to fall 3-1 behind.

Thiem faced six more break points and he erased them all but the single break was enough for Anderson to seal the first set after he forced his 25-year-old opposite into a net error.

"I didn't have a good start," said Thiem, who has now lost a third straight opening match in as many years at the Finals.

"I had no chance basically in the first set to break him. I was in trouble almost every service game. So the first set was not really good."

The second set was much tighter and with no break points offered as they entered a tie-break that was on a knife's edge.

"The second set could have been anybody's set," said Anderson as the sole South African representative in the tournament's history dating back to 1970.

"He started serving well. I was taking care of my serve games. It really could have gone either way."

An ace helped Thiem cancel out a third match point but Anderson unleashed a forehand winner to lead 11-10 before claiming the contest on his racket with his 13th ace after one hour and 48 minutes.