Top-ranked Novak Djokovic launched his campaign for a record-equalling sixth ATP Finals title with a comfortable victory against American debutant John Isner on Monday.
The Serb, guaranteed to end the year in top spot, established an early foothold in Group Guga Kuerten by seeing off the threat of the tall American 6-4 6-3.
Djokovic has enjoyed a remarkable second half of the year by winning Wimbledon and the US Open as he surged up the rankings in just five months after making an incredible comeback following elbow surgery.
And despite a blip after losing in the final of the Paris Masters to Karen Khachanov eight days ago, he returned to winning ways at The O2.
Isner, at 6ft 10in, is the tallest player to compete at the year-end championships. Despite a solid start, it was the 31-year-old who broke his resistance in the fifth game and that was enough for the Serb to close it out. He lost just four points on serve in a set lasting 43 minutes.