It took just two straight sets for Novak Djokovic to reaffirm his place as World Number One.
The Serb won 6-1 6-1 in 65 minutes, beating Japan’s Kei Nishikori to extend his winning streak to 15 at the O2 Arena, a run stretching back to 2011.
The world number one needed just over an hour to see off eighth seed Nishikori, and ended the afternoon with a trophy in his hands as he was confirmed as the 2015 world number one.
Djokovic, 28, won the Australian Open, Wimbledon and US Open, and went within one match of the calendar Grand Slam, losing to Stan Wawrinka in the French Open final.
“Undoubtedly it has been the best season and the best year of my life – I became a father and I got married. Those are the kind of trophies and milestones that mean the most to me,” he said.
Meanwhile, Roger Federer beat Tomas Berdych 6-4 6-2 in 69 minutes, in the opening day’s other round-robin match in London.
At the men’s doubles match, Jamie Murray won on his debut as he and doubles partner John Peers beat Simone Bolelli and Fabio Fognini.
All the players wore black ribbons on their shirts as a mark of respect to the victims of Friday’s deadly attacks in Paris.
“All my heart goes out to the families of the lost ones, it’s really devastating to see those images and that footage,” said Djokovic.