Johannesburg: Swiss maestro Roger Federer has warned world-number one Novak Djokovic, saying that it will be hard for the latter to repeat his glorious year like 2015.
Federer became Djokovic's last victim of the year as the Serbian claimed a 6-3, 6-4 victory in the finals of the ATP World Tour Finals.
The 34-year-old said that although Djokovic is playing great at the moment, he cannot play at the same pace all the time, Sport24 reported.
Federer, however, admitted that the coming year won't be bad for him as he is going with massive confidence.
The 17-time Grand Slam champion said that he needs to work on his overall game in order to put greater impact against players.
Although the Swiss sporting marvel had ended Djokovic's 23-match winning streak earlier this week in the group stage, the Serbian once again got the better of Federer in grand finals.
With the win, Djokovic became the first player in the tournament's 46-year history to lift the title four years in a row.
Djokovic displayed an imperious show throughout the year, winning three Grand Slam titles and taking his singles tally to 10.