Defending champion Novak Djokovic trounced stand-in David Goffin 6-1 6-2 for a clean sweep of his group at the ATP World Tour Finals on Thursday but still looked far from happy as he picked a row with the chair umpire.

Belgium's Goffin pocketed $179,000 (143,967 pound) for his brief appearance, courtesy of an injury that forced Gael Monfils to withdraw on Wednesday, earning almost $60,000 for each game he won in a no-contest lasting 69 minutes.

It was poor value for the O2 Arena crowd and nothing more than a gentle workout for Djokovic who did his best to enliven the flat atmosphere by having a lively exchange of views with umpire Fergus Murphy after being given a time violation.

Djokovic, who had already won the group after wins over Dominic Thiem and Milos Raonic, was given the warning for going over the 25-second time limit while serving at 3-1.

"You have no clue about the game," a irate Djokovic told Murphy at the changeover before turning to continue the debate with tournament director Tom Barnes.

Djokovic, whose year has tailed off since he claimed the French Open in June and was usurped as world number one by Andy Murray last week, has been prickly in London and argued with a reporter during a news conference on Sunday.

The Serb, who could snatch back the world number one ranking from Murray if he wins a fifth successive title in London, did not let the flare-up effect his progress and broke for the second time in the next game.

World number 11 Goffin was coming in cold and never really got to grips with the court surface, the unique setting or his clinical opponent and despite some occasional flashes of his silky shot-making he was never in it.

Djokovic will face the runner-up of the John McEnroe group in the semi-finals, be it Murray, Kei Nishikori or Stan Wawrinka. That group will be decided on Friday.

Later on Thursday Canada's Raonic and Austrian Thiem face off with the winner joining Djokovic in the semis.