Roger Federer staved off elimination from the ATP Finals as the six-time champion eased to a 6-2, 6-3 win over Dominic Thiem on Tuesday.

After losing his opening match at London's O2 Arena against Kei Nishikori, Federer would have crashed out if he had been beaten by Thiem.

Only once before has Federer failed to qualify from his group for the knockout rounds in his 15 previous appearances at the prestigious season-ending event.

That lone flop came 10 years ago when the Finals were held in Shanghai and Federer is still in with a chance of avoiding that ignominious fate thanks to his comfortable 66-minute dismissal of Thiem.

Bidding for a record seventh ATP Finals crown and a first in the tournament since 2011 — which would be the 100th singles title of his glittering career — world number three Federer must beat South Africa's Kevin Anderson in his last group match to make the semi-finals.

"It feels good. I am very happy that I showed a reaction after the last match," Federer said.

"Against Kei, instead of seeing things positively, I thought negatively. It was disappointing but it happens.

"Today I was more positive and happy on the court. I love playing in London and I had to remind myself what a privilege it is. I hope I showed it."

Since winning his 20th Grand Slam title at the Australian Open in January, Federer has been stuck in a prolonged slump by his sky-high standards.

A quarter-final exit at Wimbledon, blowing a two-set lead against Anderson, was followed by a stunning US Open last 16 defeat to Australian journeyman John Millman.

The 37-year-old's defeat in his opening Finals match against Nishikori on Sunday marked the first time Federer had lost in straight sets in 46 group stage matches.

He responded to that setback by cancelling his practice session on Monday, sparking fears he was seriously hampered by a hand injury that has troubled him since the grass-court season in June and July.

Having cut a grumpy figure during his error-strewn loss to Nishikori, Federer's spirits were lifted by this solid show, aided by Thiem's 34 unforced errors.

After missing a break point in the opening game, Federer started to find his rhythm and he landed the first break for a 2-1 lead when the world number eight sent a forehand long.

Another break followed in the seventh game as Thiem netted a tame volley as Federer won the first set with ease.

While it wasn't vintage Federer, he was doing enough to take advantage of Thiem's sloppy display as the French Open finalist double-faulted to gift a break in the opening game of the second set.

That was all the encouragement Federer needed as he cruised to the finish line.

Zverev in dilemma

Novak Djokovic moved closer to a place in the semi-finals of the ATP Finals as the world number one dispatched Alexander Zverev 6-4, 6-1 on Wednesday.

Djokovic is chasing a sixth ATP Finals title and his first in the prestigious season-ending event at London's O2 Arena since 2015.

In his 11th appearance at the Finals, the Serb will be guaranteed to reach the last four for the eighth time if John Isner beats Marin Cilic or if Cilic defeats Isner in three sets later on Wednesday.

Djokovic, who beat Isner in his opening group-stage match, has enjoyed a remarkable renaissance in the second half of 2018 and lifting the trophy on the banks of the River Thames would be the perfect way to finish his year.

Djokovic faces Cilic in his last group match on Friday, while Zverev, who defeated Cilic to kick off his Finals campaign, takes on Isner.

The 31-year-old was assailed by claims he had lost his hunger for success and was past his best after injury and personal issues threatened to derail his career in 2017.

But Djokovic has banished his demons in spectacular style, winning the Wimbledon and US Open titles, returning to world number one and becoming the first player to win all nine Masters 1,000 tournaments.

"I found it more difficult than in the first match against John Isner. I played well midway through the second set and started to swing through the ball," said Djokovic, who has suffered only two defeats in the Finals since 2012.