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.

The Serbian world number one produced a ruthless display against a ragged US 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.

Earlier, Wawrinka returned to form with a 6-1, 6-1 thrashing of Berdych.

The Australian Open champion has endured a dismal run of results since the US Open, winning only one Tour match since then, but the Swiss looked back to his best as he opened proceedings in Group A at the O2 Arena.

He came within a point of winning the opening set 6-0 but was thwarted by a Berdych ace.

His opponent could not turn the tide, however, and last year's semi-finalist Wawrinka wrapped up victory in 58 minutes.

Source: Reuters