Roger Federer kept alive his hopes of a seventh ATP Finals crown on Tuesday, beating Italian debutant Matteo Berrettini 7-6 (7/2), 6-3 at London’s O2 Arena.

The Swiss third seed put himself under enormous pressure by losing his first match on Sunday in straight sets in Group Bjorn Borg to Dominic Thiem.

The six-time champion was not at his fluent best on Tuesday but ultimately had too much for Berrettini, who won just three games against Novak Djokovic in his opener.

Both players were solid on serve in the first set, with Federer struggling to make inroads against the eighth seed.

The Swiss eventually earned himself the sniff of a chance in the 12th game, winning the first break point of the match but Berrettini snuffed out the danger and held to take it into a tie-break.

Federer upped the level of his game in the shootout, helped by some wayward shots from his opponent plus a Berrettini double-fault and won it comfortably 7-2.

The Swiss, clad in black, broke immediately at the start of the second set to leave the Italian with a mountain to climb.

Berrettini earned a clutch of break points in the eighth game but Federer eventually served himself out of trouble.

Federer, as usual enjoying the lion’s share of support from the crowd, broke in the next game to seal the set 6-3.

The 20-time Grand Slam champion said he had remained calm and tried to stay positive after the Thiem defeat.

“There’s no reason to be too down on yourself,” he said. “We came here to play three matches and give it all we have. It was the big goal of the season to come here and qualify which we did, plus I had a day off.”

“Not everything was bad (in the Thiem match) but of course if you over-analyse it, all of a sudden it can be,” he added. “I was ready, I was prepared today and that’s what matters the most.”

Earlier, Rafael Nadal refused to make excuses after being outplayed by defending champion Alexander Zverev on Monday.

The Spanish top seed, who has never won the event, came into the tournament at London’s O2 Arena under an injury cloud and was well short of his imperious best as he went down 6-2, 6-4.

In the earlier round-robin match in Group Andre Agassi, sixth seed Tsitsipas beat Daniil Medvedev 7-6 (7/5), 6-4, admitting their spiky relationship had made his job tougher.

Nadal is locked in a battle with Novak Djokovic to finish as the year-end number one but was not serving flat out in practice last week due to an abdominal strain that forced him to pull out of the Paris Masters at the semi-final stage.

The 33-year-old led Germany’s Zverev 5-0 in head-to-head contests coming into the match but was uncharacteristically sloppy in the first set, conceding two breaks of serve.

Buoyed by his dominant start, Zverev, seeded seventh, broke in the first game of the second set to take an iron grip on the match.

Nadal dug deep, urging himself on but his 22-year-old opponent proved too strong as he served out the set. He did not concede a single break point in the entire match.

Zverev hit a total of 26 winners -- double his opponent’s tally -- and Nadal managed just three forehand winners in the entire match.

Zverev, one of four players under the age of 24 at the event in London, was delighted to recapture his best form after a mixed season.

“This means so much, playing here again after winning my biggest title so far in my career here last year,” he said. “This means everything to me.

“Playing here, playing in front of you all, playing in the O2 is something that we don’t have during the year, and this is so special.”

Nadal faces a tough task now to qualify for the semi-finals from the group, with matches still to come against Tsitsipas and Medvedev.

The Spaniard has qualified for the year-end championships for 15 years in a row but has only made eight prior appearances due to injuries.

Agence France-Presse