Roger Federer has warned Novak Djokovic that he is "working hard" to end his dominance of ATP Tour circuit.
World number one Djokovic enjoyed a stunning 2015 season, claiming three grand slam titles and only being denied a clean sweep by an inspired Stan Wawrinka in the French Open final.
He also won a record six Masters 1000 tournaments and finished the year with a scarcely believable 82-6 win-loss record by beating Federer in last month's World Tour Finals showpiece at London's O2 Arena.
Djokovic was the primary reason the evergreen Federer failed to add to his haul of 17 slams in 2015, with the Serb defeating the Swiss in the finals of both Wimbledon and the US Open.
But Federer - who appointed Ivan Ljubicic as his new coach this week - is determined to beat his rival to tennis' biggest prizes next year.
"Novak was really tough to beat, especially on the big occasions," said Federer, in quotes reported by The National ahead of his Indian Premier Tennis League meeting with Rafael Nadal on Saturday.
"He's got sky-high confidence and we hope that will come down again sooner than later, but we're working hard to try to catch him."
When quizzed on what the secret to Djokovic's success is, Federer replied: "He just became a really, really good player and tough to beat. Especially in recent years, he's taken it to another level.
"That just made it harder for me to dominate him. But still I'm winning my fair share of matches against him, and I'm really enjoying playing against Novak."
Federer versus Nadal was the rivalry that defined men's tennis for much of the previous decade and the 34-year-old says the Spaniard - who holds a 23-11 record over him - has represented "the ultimate challenge" throughout his career.
However, Federer refused to be drawn over who is the superior player - Djokovic or Nadal.
"Totally different players, but Rafa's been tougher for me even though I guess Novak has beaten me as many times as well," he added.