Novak Djokovic made it three wins from three at the Barclays ATP World Tour Finals with victory over alternate David Goffin.

The Belgian stepped in after Gael Monfils pulled out on Wednesday with a rib injury to become the third debutant at this year's tournament.

But it was not a happy experience for Goffin, who detained Djokovic for just an hour and nine minutes before the Serbian wrapped up a 6-1 6-2 triumph.

Djokovic said: "I felt most comfortable (on Thursday) of all the matches I've played. I felt like I increased the level of the quality of tennis, so that's a good sign.

"I was preparing to play Gael and then (on Wednesday night) I found I was going to play David. It's hard for him to come out here, he didn't play his best tennis."

Djokovic was already guaranteed to top the group while Goffin had no chance of progressing to the semi-finals, but there was still the small matter of nearly £150,000 and 200 ranking points at stake.

Djokovic had survived stern tests from Dominic Thiem and Milos Raonic but, from the moment he broke to lead 3-1 in the opening set, this was a one-sided contest.

The only real moment of unhappiness for the defending champion, who has lost just one match here since 2011, came after he received a time violation in the following game.

Novak Djokovic argues with the match umpire during his victory over David Goffin (Getty)

Djokovic was furious with umpire Fergus Murphy and also made his feelings known to the tournament supervisor at the change of ends.

It did him no harm on court, though, with Djokovic quickly wrapping up the first set and forging ahead in the second.

Djokovic and Goffin shake hands after their group match at the ATP World Tour Finals (Getty)

Goffin had moments of brilliance but they were all too few and far between, and the support of a crowd desperate for a better match could not help him change the momentum.

The Belgian created his first break point in the final game of the match but could not take it and Djokovic will now wait to see which of Andy Murray, Stan Wawrinka or Kei Nishikori he will meet in the semi-finals on Saturday.