"Pick a side," Djokovic said about beating one of the biggest servers in the men's game.

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Djokovic seeking record-tying sixth title at season-ending ATP Finals

Serb regained No. 1 spot shortly before ATP Finals

London, UK CNN  — 

After a shock loss by Roger Federer on Sunday, order was restored at the ATP World Tour Finals on Monday by Novak Djokovic.

Gliding across the court of London’s O2 Arena as if he had never been away, the Serb neutralized the booming serves of Wimbledon marathon man John Isner with a commanding 6-4 6-3 win in the group stages of the season-ending ATP Finals for the top eight players in the world.

“The match was great, obviously, to have three breaks of serve of John, which is sometimes mission impossible,” Djokovic, who struck 21 winners and took 42% of his return points, told a packed crowd, which included soccer star Cristiano Ronaldo, at London’s O2 Arena.

When asked how you play a return game against a player who has struck close to 11,000 aces in his career, Djokovic said: “Make sure you are as far from the baseline as possible. Pick a side,” he said.

Tying Federer

The 31-year-old Serb, who didn’t play in London last year because of injury, is trying to win the ATP Finals for a sixth time, tying the record held by Federer.

The Swiss, who is chasing his 100th career title, had opened the ATP Finals on Sunday with a 7-6 (7-4) 6-3 defeat by the ninth-ranked Kei Nishikori of Japan. Federer, 37, plays his second group match against French Open finalist Dominic Thiem of Austria on Tuesday night.

This time last year, Djokovic had been forced to sit out the ATP Finals as he tried to recover from an elbow injury, for which he eventually underwent surgery in February.

Tough start to 2018

Unlike Federer, who returned from a six-month injury break at the start of 2017 to win the Australian Open, Djokovic struggled during the first few months of his comeback, even falling out of the top 20 in May for the first time since 2006.

After ending his coaching relationship with former world No. 1 Andre Agassi, Djokovic reunited with his old team led by Marian Vajda in April. Although his form improved, he crashed out of the Roland-Garros quarterfinals in a tempestuous match against world No. 72 Marco Cecchinato of Italy, and even hinted at skipping the grass-court season altogether.

Isner eyes future grand slam
04:24 - Source: CNN

But he regrouped after Paris, and has since gone on a tear: winning Wimbledon and the US Open and ATP World Tour titles in Cincinnati and Shanghai before regaining the No. 1 spot last week.

“It has been great to be back at the O2 Arena,” said Djokovic, who will end the season as the world’s top-ranked male tennis player for a fifth time. “I love playing in this place, it has been a lucky place for me over the years.”

READ: Why Serena Williams’ coach is calling for on-court coaching

READ: Rafa Nadal out of ATP Finals with injury

Return masterclass

It wasn’t that Isner, who had averaged 23 aces per match in the past year, played badly.

In fact, some of his serves were so fast, they forced the crowd on the lower tiers of the O2 Arena to duck.

The 33-year-old American, the oldest ATP Finals debutant since 1972 and at 6-foot-10 the tallest player to ever have competed in the championships, struck 13 aces and hit 69% of his first serves in.

South Africa's Kevin Anderson (R) shakes hands after beating Isner in their six-and-a-half hour Wimbledon epic earlier in the year.

But Isner, a Wimbledon semifinalist this summer, had no answer to the precision of Djokovic’s return game.

Watched by Ronaldo and his family, Djokovic forced the first break of the match at 2-2 with a pinpoint forehand return. After he served out the first set to love, 6-4, Djokovic broke again at 3-3 in the second set.

Serving to stay in the match at 5-3 down, Isner was broken for the third and final time as Djokovic found another backhand passing shot for a winner.

“He’s the best returner I’ve ever faced for sure,” Isner said.

READ: The ‘Bromance’ between John Isner and Kevin Anderson

Zverev next

Earlier on Monday, Germany’s Alexander Zverev defeated former US Open winner Marin Cilic of Croatia 7-6 (7-5) 7-6 (7-1) in the other round-robin match in the group Guga Kuerten.

Djokovic will next play the big-serving Zverev, with whom he is tied 1-1.

“I’m feeling fit physically and mentally,” said Djokovic, who lost to Zverev last year on clay in the finals of Rome but beat the 21-year-old in the semifinals of Shanghai last month. “I’m looking forward to that match.”

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He added: “I played him recently in Shanghai, but wasn’t at his best. I expect him to play better than he did in Shanghai in two days.”