Novak Djokovic began his challenge for a hat-trick of ATP World Tour Finals triumphs in London with a 6-1 6-1 demolition of Croatia’s Marin Cilic on Monday.

The Serbian world number one produced a ruthless display against a ragged U.S. Open champion, opening his Group A account with a 56-minute masterclass.

The 27-year-old has now won 28 consecutive matches indoors dating back to 2012 and should he win his remaining two group matches against Stanislas Wawrinka and Tomas Berdych he will be guaranteed to end the year on top of the rankings.

Cilic won the opening five points of the match but his debut at the ATP’s prestige event quickly turned sour as Djokovic turned up the heat with a clinical display.

The only blemish was dropping his serve early in the second set but even then he broke again immediately to snuff out any chance of a Cilic fightback.You have to go back to the Paris Masters in 2012 for the Serb’s last defeat under a roof when he lost to American Sam Querrey. Since then he has been untouchable.

Federer beats Nishikori

Roger Federer produced another masterly display to beat Japan’s Kei Nishikori 6-3 6-2 and move a step closer to the semi-finals at the ATP World Tour Finals on Tuesday. The six-times former champion was at his imperious best to subdue the 24-year-old U.S. Open runner-up in one hour and nine minutes.

The 33-year-old Federer, who can still knock Novak Djokovic off the top of the rankings before the end of the year, fought off two break points in the third game, one with an ace, and broke himself a game later with a flowing forehand pass. That was enough for the Swiss maestro to secure the opener and the quality of his play pressured Nishikori into several errors on the backhand side at the same stage of the second set.