A merciless Djokovic brutally dismantled the reigning US Open champion and tournament debutant 6-1, 6-1 in just 56 minutes, producing a carbon copy of Swiss star Stan Wawrinka’s success over Tomas Berdych earlier in the day.
World No1 Djokovic is aiming to becomes the first man since Ivan Lendl (1985-87) to win the season-ending event three times in succession and looked in ominous form as he extended his unbeaten run indoors to 28 matches.
“It was a great match,” said Djokovic. “Marin is a US Open winner just a few months ago and has a lot of confidence but I managed to neutralise his serve and managed to get a lot of balls back in play and that was part of the tactics.
“I’m not guessing. Most of the time it is the practice which makes it perfect and a lot of hours spent on court trying to get my game, my anticipation and my return to the level that I want.
“Those are elements in the game that I can rely on always and, for sure, it is very useful when you’re playing a big server.”
Djokovic joins Wawrinka at the top of Group A, with the Australian Open champion recording a crushing 6-1, 6-1 defeat of Berdych in another one-sided clash which also failed to last an hour.
Berdych said: “Unfortunately, it was my worst match of the whole season. My game is about hitting the ball nice and clean, then you can create something but at the beginning I didn’t have that at all. I hit so many frames.”
ORDER OF PLAY
■ Roger Federer  (SWI) v Kei Nishikori  (JPN). Not before 2pm
■ Andy Murray  (GBR) v Milos Raonic  (CZE). Not before 8pm