World number one Novak Djokovic underlined why he is favourite to win the ATP Finals with a commanding win over John Isner in their group opener.

The Serb, 31, was in devastating form as he easily dealt with the big serves of American Isner in a 6-4 6-3 victory.

World number 10 Isner, making his debut at the season-ending tournament, was broken in the fifth game of the first set and seventh of the second.

Djokovic then sealed the win on Isner’s serve with a crosscourt backhand.

Earlier in the day, Alexander Zverev beat Marin Cilic in straight sets in the other match in the Gustavo Kuerten group.

• Zverev punishes Cilic at ATP Finals
• Watch: Five best shots as Zverev beats Cilic

From favourite, to hot favourite

With world number three Roger Federer having slumped to a surprise defeat by Kei Nishikori in his opening group match on Sunday, the path for Djokovic to a sixth title at the tournament seems ever clearer.

He was already helped by the withdrawal through injury of world numbers two and four Rafael Nadal and Juan Martin del Potro and after this performance he is an even hotter favourite.

An encounter between the leading server on the Tour in Isner and the best returner was an intriguing prospect, but it was soon clear who would have the upper hand.

Isner is a player synonymous with tie-breaks – so impenetrable is his serve – but Djokovic already had him under pressure in the opening game with a break point.

Although he netted his shot to squander the chance, and then missed another in the third, he made no mistake in the fifth game when he broke with a beautiful baseline return from yet another thundering first serve from the American.

Djokovic’s own serving was impeccable, winning six of his service games to love, to give Isner no chance of a breakthrough.

He sealed his first break in the second set when Isner netted a forehand in the seventh game and then set up three match points on Isner’s next service game – eventually wrapping up victory on the third one with a fantastic backhand crosscourt winner after one hour 13 minutes.

“I had three breaks of serve of John which is sometimes ‘mission impossible’ but I managed to be at right place at the right time,” said Djokovic.

“I held serve well, I backed it up from the baseline, I played very solid and didn’t give him many opportunities.”

How Djokovic out-served a server

Isner, who at 33 is the oldest debutant at the season-ending finals since Andres Gimeno in 1972, had qualified for the tournament as a result of Nadal and Del Potro’s withdrawals.

He has enjoyed his most successful year – reaching his maiden Grand Slam semi-final at Wimbledon, winning his first Masters 1000 title at Miami and finishing the season ranked inside the world’s top 10 for the first time.

And he was not overawed by his first appearance on this stage – still managing to deliver 13 aces and serves of more than 140mph, including some second serves reaching more than 125mph.

But the most telling number was the 66% of first-serve points won by a player whose average for the season is a huge 81%.

That is testament to the returning of Djokovic, who kept reading the serves and getting his racquet to whatever was thrown at him time and again.

And the Serb’s own percentage of first-serve points won was 86%, which is more the kind of figure Isner is used to posting.

Analysis

GB Davis Cup captain Leon Smith on 5 live sports extra

Novak Djokovic is as good as ever. It is a privilege to watch that.

What he is doing against one of the best servers in the world, to neutralise him and turn defence into attack, dominating back of the court, it is phenomenal.

He is in a good place, physically and mentally, his timing of the ball is fantastic and he’s oozing confidence.

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.