Novak Djokovic sent out a warning to his rivals at the ATP Finals on Monday, brushing aside big-serving John Isner 6-4, 6-3 to launch his bid for a record-equalling sixth ATP Finals title in style.

World number one Djokovic, though, was playing a different game, appearing little troubled by the howitzers coming off the giant Isner's racquet -- breaking his opponent three times and not conceding a single break point on his own serve.

In stark contrast to second seed Roger Federer, who produced an error-strewn performance in defeat to Kei Nishikori on Sunday, Djokovic hit just six unforced errors and won 86 percent of points on his serve, making a mockery of the apparent challenging nature of the playing surface.

"The match was great, obviously," said the 31-year-old. "I had three breaks of serve of John which is sometimes mission impossible but I managed to be at the right place at the right time.

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

Speaking about the court conditions, he said: "It takes a little bit of time really to get yourself adjusted to the surface... It takes a lot of rotation, takes a lot of spin. When you serve well, also it accelerates through the court."

With Cristiano Ronaldo watching on, the top seed broke Isner in the fifth game of the first set to establish a stranglehold and never looked like allowing the American eighth seed back into the contest, breaking twice more in the second set to win in 73 minutes.

The Serb is a red-hot favourite to draw level on six titles at the season finale with Federer, who faces an uphill task to reach the semi-finals after his loss in his first round-robin match.

Djokovic, who replaced the injured Rafael Nadal at the top of the rankings last week, has stormed up the charts this year following elbow surgery and a period of indifferent form, winning Wimbledon and the US Open along the way. Since the start of Wimbledon he has now won 32 out of 34 matches.

anderson STARS

Kevin Anderson handed Japan's Kei Nishikori a humiliating 6-0, 6-1 thrashing at the ATP Finals as the South African moved to the brink of a semi-final berth on Tuesday.

Anderson won the first 11 games and needed only 64 minutes to demolish the woeful Nishikori in a remarkably one-sided group stage clash at London's O2 Arena.

Having also won his opening match against Dominic Thiem in his debut at the prestigious season-ending event, Anderson is close to becoming the first African to qualify for the last four at the ATP Finals.

The 32-year-old's place will be secure if Thiem beats Roger Federer, or if Federer defeats Thiem in three sets later on Tuesday.

It was a landmark moment for Wimbledon finalist Anderson, who has set a new career-high of 47 victories in a calender year.

He will rarely have had such an easy outing as this one throughout his memorable 2018.

Anderson won the Vienna final at Nishikori's expense in October before losing to the Japanese star in the Paris Masters four days later.

But the close nature of those matches was a distant memory as Nishikori, who had beaten Federer in straight sets on Sunday, put his hopes of a semi-final spot in jeopardy with this tame surrender.

Anderson's final group match is against Federer on Thursday, while Nishikori faces Thiem.

It has been a curious year for Nishikori as the former US Open finalist, troubled by a wrist injury, started 2018 losing to the world number 238 in a Challenger event in January and crashed to 39th in the ATP rankings in April after a prolonged slump.

But the 28-year-old had climbed back to ninth in the world thanks to a strong second half of a campaign, featuring a semi-final appearance at the US Open.

His renaissance peaked when he handed former ATP Finals champion Federer his first straight sets defeat in the group stage of the tournament.

But Nishikori seemed to have nothing left after that effort.

Anderson is back in London just four months after losing his first Wimbledon final against Novak Djokovic.

And he didn't take long to seize the initiative as he broke Nishikori in the second game of the match.

Unloading some searing ground-strokes, Anderson, aided by 14 unforced errors from Nishikori, cemented his advantage with two more breaks in the fourth and six games.

It had taken Anderson just 31 minutes to complete his first set masterclass and he didn't take his foot off the gas.

With Nishikori remaining out of sorts with another 10 unforced errors in the second set, Anderson powered ahead to make it 11 games in a row.

Nishikori avoided a whitewash when he won the 12th game to ironic cheers from the crowd, but Anderson quickly closed out the win.

Earlier in doubles, Bryan and Sock won their opening match against Lukasz Kubot and Marcelo Melo 6-3, 7-6 (7/5).

"It kind of hit me this week, practising but I can't complain," said Sock, when asked if it felt strange to be back at the O2 playing doubles rather than singles.

"It's so different being here from last year, top eight in the world in singles," he added.

"Obviously I was on a high after Paris and it was an unreal finish to the year... hopefully I can be back here in singles next year.”