Novak Djokovic, the globe's top-ranked tennis player, beat Roger Federer 7-6, 7-5 here Monday to win his second Barclays ATP World Tour Finals.

Displaying again his vaunted mental toughness, the Serb battled back after falling behind in the first set and denied the Swiss great what would have been his seventh triumph in the year-end event.

Federer, who came into the final at a packed O2 Arena in London having beaten Djokovic in 16 of their 28 prior confrontations, jumped out to a 3-0 lead in the first 10 minutes of the opening set.

The unrattled Djokovic leveled the contest at 3-3, but was unable to execute a set point at 5-4 and Federer forced him to a tiebreaker.

The Swiss player likewise missed out on a set point at 5-4 in the second set and was unable to respond when Djokovic took a 6-5 lead.

No matter what happened in the final, Djokovic was assured of finishing as the world's top player for the second consecutive season.

The eight-man Barclays ATP World Tour Finals is the biggest tournament on the men's tennis calendar after the four Grand Slams.