Roger Federer, who lost in the final of the ATP World Tour Finals to Novak Djokovic, says that the Serbian will still be hard to beat in 2016.
Federer went 3-5 in matches against the world No. 1 this year. Their career head-to-head tally is tied at 22-all.
"It's hard to play at this pace all the time, doesn't need much," 17-time Grand Slam champion Federer said. "We're talking margins. You don't win a breakpoint, he does, vice versa, it changes the whole outcome of the match. You can't always be on the winning side. Margins are small at the very top.
"That's why this year of Novak's is amazing. Rafa [Nadal] has been there. I've been there. We both know how hard it is to back it up year after year."
Djokovic won 11 titles this season, including three Grand Slams and six ATP 1000 Masters. Federer won six titles this season, but was unable to win a major and won just one ATP 1000 Masters title (in Cincinnati).
Federer said the 10-time Grand Slam champion could repeat his performance in 2016, but it will be a challenge.
"It's not the first good year of Novak. Clearly he's going into next year with massive confidence. He's playing great. It's going to be hard for him to play a bad year, that's for sure. But this extremely good, it's always tough," the 34-year-old said. "But right now you would think it is somewhat possible, but you can't just repeat a year like this. It takes a lot of effort. You've got to be physically in shape, no injuries whatsoever. Mentally, you have to be at your peak at all times. It's not as easy as it seems sometimes."