LONDON, Nov. 12 (Xinhua) -- Six-time champion Roger Federer defeat world No. 8 Jack Sock in straight sets in the ATP World Tour Finals opener at the O2 Arena here on Sunday.

The 36-year-old Federer broke the American in the first game and went on to take the first set 6-4 in just 36 minutes.

Sock, who had guranteed a place in the season-ending event for the first time by winning the Paris Open, stepped up offensive in the second set, forcing it into the tiebreak.

Federer, ranked second after Rafa Nadal, fought back from 1-0 down to seal the victory as the 25-year-old made too many mistakes including a double-fault at the crucial moment.