(CNN) -- Novak Djokovic completed his second three-set victory over Roger Federer in the space of three days to get his ATP World Tour Finals title defense off to a winning start Tuesday in London.
Djokovic had beaten Federer in the semifinals of the Paris Masters Saturday and again had to fight off a determined challenge from the Swiss legend in another compelling encounter.
The Serbian World No.2 eventually won 6-4 6-7 6-2 in two and a half hours at the O2 Arena to top Group B ahead of Juan Martin del Potro, who won his three set opener against Richard Gasquet Monday.
Federer, making his 12th straight apperance at the end of season showpiece, was cheered to the rafters when he made his entrance and then set about showing that at 32 years of age he was still a force.
He dropped the first set to a single break in the 10th game, but then twice pulled ahead in the second as he made inroads into Djokovic's delivery.
Each time the defending champion broke back, but in the tiebreak Federer was inspired and with the decibel level getting every higher he leveled at a set all.
But Djokovic immediately broke him and controlled the decider to wrap up a clinical victory.
"It was a big challenge physically. Only 48 hours ago, I was playing in Paris so to be able to beat Roger is incredible," Djokovic told gathered reporters.
"I'm going to try and recharge my batteries mentally and physically and get ready to play Juan Martin del Potro."
It was his 18th straight win since losing in the final of the U.S. Open to Rafael Nadal, with titles in Beijing, Shanghai and Paris.
Earlier, Nadal beat fellow Spaniard David Ferrer in their opening match -- reversing the result of the semifinal at the Paris Masters Saturday.
Nadal won 6-3 6-2 to begin his campaign for an elusive title at the end of season tournament in fine style.
He will clinch the No.1 spot for 2013 with victory over Stanislas Wawrinka of Swtizerland in the opening match on court Wednesday in Group A.
Nadal needed only one hour 14 minutes to seal victory -- a run of seven straight games the key to his win.
"He played a lot better than me in Paris last week but today I played a little bit better than him and it's important for my confidence to start with a victory after Paris.
"The energy of this stadium is fantastic and hopefully I am ready for tomorrow against Stanislas Wawrinka," said Nadal, who is returning to the 02 Arena after a two-year break.