Novak Djokovic admitted he had to dig deep to beat Roger Federer in his ATP World Tour Finals opener.
Defending champion Djokovic, just two days after lifting the Paris Masters, saw off Federer 6-4 6-7 (2/7) 6-2 in Group B at the season-ending tournament in London.
The Serb told Sky Sports: "It is necessary to be on top of your game and perform your best if you want to win against Roger.
"He has a terrific record in London, winning so many Wimbledon titles and here several times. He loves playing here.
"It was big challenge physically for me. Only 48 hours ago I was playing in Paris, so to be able to play this well and win against Roger in the opening group match is incredible."
Djokovic's next match of the round-robin is against Juan Martin del Potro on Thursday.
"First I have to enjoy the win and then think about recovering," said Djokovic.
"It's a big win at the beginning of this year's World Tour Finals and the good thing is there is a day off and I'm going to try to recharge my batteries, mentally and physically, and get ready for Del Potro.
"It doesn't get easier: Roger and then Del Potro. The quality of players taking part in this tournament is the highest possible in tennis, the best eight players in the world. Every match is high quality.
"That's why you need to be physically ready and hopefully I can maintain this rhythm and play my best against Juan Martin."