Roger Federer cancelled a practice session on Monday before he faces Austria’s Dominic Thiem in a crucial round-robin match at the ATP Finals.

The 37-year-old Swiss is likely to be eliminated if he loses in Tuesday’s match at the O2 Arena (20:00 GMT).

The six-time champion lost his opening match to Japan’s Kei Nishikori in straight sets on Sunday.

He said afterwards he was struggling to find a practice court in London which matched the speed of the O2.

Making his 16th appearance at the season finale, the world number three had not lost any of his previous 45 round-robin matches in straight sets before his 7-6 (7-4) 6-3 loss to Nishikori.

“Practice was a bit all over the place, practising at Queen’s, practising on outside courts here and at Centre as well,” he said after Sunday’s match.

“So it was not always the same conditions.”

World number one Novak Djokovic is the favourite to win the event, boosting his credentials with an impressive opening win against John Isner on Monday.

The top two players in each group qualifying for the semi-finals on Saturday, 17 November.

The winners meet in the final on Sunday, 18 November at 18:00 GMT.

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Germany’s Alexander Zverev punished Croatian rival Marin Cilic’s inability to take his chances and opened his ATP Finals campaign with a 7-6 (7-5) 7-6 (7-1) win at the O2 Arena in London.

Cilic, 30, served for the first set but made three unforced errors and a double fault as Zverev broke back for 5-4.

Zverev, 21, went on to take the tie-break and then fought back from a break down midway through the second set.

Cilic saved a match point at 5-4, but Zverev clinched victory in the breaker.

Zverev, like he did in the first-set tie-break, raced into a comfortable lead in the second decider, taking the second of five more match points with a serve out wide which Cilic could not return.

World number one Novak Djokovic leads Zverev at the top of Group Gustavo Kuerten after beating American John Isner in Monday’s evening match at the season-ending tournament.

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Can Zverev shine on one of the bigger stages?

With the ‘big four’ of Roger Federer, Rafael Nadal, Novak Djokovic and Andy Murray all into their thirties, Zverev has long been identified as the young player most likely to fill the void when they eventually retire.

However, his potential is yet to be realised at the highest level, with only one appearance in a Grand Slam quarter-final to show so far in his developing career.

His game has proved to be most adept on clay courts, with two of his three ATP Masters victories – as well as that Slam quarter-final at this year’s French Open – coming on the slower surface.

Before August’s US Open, Zverev brought eight-time Grand Slam champion Ivan Lendl into his team to help him go deeper in the four majors – and he showed his mental resilience against Cilic on Monday at a tournament considered second only to the four Slams in terms of prestige.

Zverev, without reaching his highest level against Cilic, cut out the errors in the key moments and took control of the second tie-break to win in two hours and six minutes.

“That first set was key because against Marin you never want to fall behind. He’s such a hard-hitting, aggressive player, when he gets confidence it is tough to play against him,” Zverev said.

“I wanted to fight back from every score, he was up a break in both sets.

“I knew it was a very important match, you don’t want to start the group stage with a losing record. it was very important to get the win.

“I’m happy with how I played and happy to build from here.”

Cilic still not feeling at home in London

Despite reaching Grand Slam finals and cementing his place in the world’s top 10 over the past couple of years, Cilic has never managed to make an impact at the end-of-season finals.

One win in his three previous appearances at the O2 is a meagre return for a player of his undoubted ability and he was hoping to improve his fortunes with an opening win against Zverev.

Yet he paid the price for a sloppy display against the world number five in a match which lacked real quality on either side of the court.

After digging deep to hold serve in the first game, Cilic was rewarded for his determination by breaking in the second but missed three chances to go a double break up at 4-0 and 5-1.

Ultimately that proved costly for the world number seven.

Still he was left serving for the set at 5-3, only to gift the break back to Zverev with three unforced errors and a double fault, allowing the German to take the first set in the tie-break.

A low first-serve percentage and more unforced errors continued to blight Cilic’s progress in the second, despite him moving a break up for 4-3 after saving a break point in the previous game with a 137mph serve.

He handed the advantage back to Zverev in the next game – and that proved decisive.

‘Great match for Zverev to come through’ – analysis

Tim Henman, former British number one and BBC Sport analyst:

There were a lot of errors in that match, it seems like the ball is difficult to control on this court.

I think there were 80 unforced errors between them and Cilic made more, he was up to about 45 errors.

When it came to the business end of both sets Zverev was able to cut down the unforced errors.

His first-serve percentage was a bit higher – around 60% – whereas Cilic was under 50%, and for a guy whose serve is a weapon that is not good enough.

For Zverev, it was a great match to come through because he did not play particularly well.