Novak Djokovic sweeps past Marin Cilic at World Tour Finals

World number one Novak Djokovic is unbeaten at the O2 since 2011

Novak Djokovic swept past US Open champion Marin Cilic on a day of one-sided matches at the ATP World Tour Finals on Monday.

The world number one began his campaign for a third straight title at the O2 Arena with a 6-1 6-1 win in 56 minutes.

Djokovic joins Stan Wawrinka at the top of Group A after the Swiss earlier thrashed Tomas Berdych by the same score.

The two singles matches required a combined one hour and 54 minutes.

"Realistically speaking, maybe we both were favourites in our matches," said Djokovic.

"But to win 6-1 6-1 in both is maybe something that people didn't expect - and us too."

The win extends Djokovic's unbeaten run indoors to 28 matches

Asked about his own victory, he added: "It was a great performance. I was hoping I could play this way.

"I was preparing myself for this match. I knew already one week ago that I'm going to play Marin at 8pm on Monday.

"My team did good scouting. We thought about what's the game plan. I stepped in and executed really well.

"His debut in the World Tour Finals got the better of him in terms of his nerves."

That Djokovic won the second match of the day was hardly a surprise, such is his dominance at the venue and over the opponent.

He is unbeaten at the O2 since 2011, has won his last 27 matches indoors and went into Monday's encounter with a 10-0 record against Cilic.

The Croat had taken sets off Djokovic three times in 2014 and must have felt optimistic after the seven-time Grand Slam champion edged through a nervy first service game.

A little over half an hour later, Djokovic had reeled off seven straight games with his familiar combination of brilliant athleticism and precision hitting.

Two running forehand passes helped the Serb break for 4-1, a third break gave him the set, and Cilic looked an understandably dispirited figure when he netted a forehand to go 2-0 down in the second.

The following game provided a reminder of the Cilic who won the US Open, as he crushed two forehands to break for love, but the comeback was quickly snuffed out.

Djokovic ratcheted up the pressure once again, peppering the baseline with his returns, and a desperate Cilic could not win another game.

"I felt pretty relaxed, not too nervous, coming into the court," said Cilic.

"Novak played really, really solid today. In some matches, the score just keeps running. You are sinking a lot. You are always trying to find something but whatever you try, it's not working."

Cilic has now lost all 11 matches he has played against Djokovic, with four defeats coming this year

Djokovic is hoping to become the first man since Ivan Lendl to win three successive ATP Finals titles

Djokovic's textbook returning restricted Cilic to just one ace in the match

US Open champion Cilic won just 23% of points on his second serve