As a former World No. 1 who has dominated the sport of tennis during his prime, Roger Federer has sound advice for current top star Novak Djokovic, who is in his own dominating run in the past two ATP seasons.

"It's hard to play at this pace all the time. You are talking about narrow margins, a break point here and there can change the whole outcome,” Federer said to Agence France-Presse via NDTV Sports. "You can't always be on the winning side. Margins are very small at the top.”

Federer went on to compare Djokovic’s impressive run in 2015 with his own performance in previous years when he had a great year and that of Spanish rival Rafael Nadal.

"That is why this year of Novak's is amazing. I've been there, Rafa (Nadal) has been there. We both know how hard it is to back it up year after year," Federer added.

While Federer believes 2016 would offer a different and tougher challenge, the former World No. 1 believes Djokovic has the “massive confidence” going into the next ATP Season.

"It's going to be hard for him to have a bad year, but you can't just repeat a year like this,” Federer said. “It takes a lot of effort. You've got to be in shape physically with no injuries whatsoever. Mentally you have to be at your peak at all times. It's not as easy as it seems sometimes."

Nadal, who is currently ranked World No. 5, spent time as the top player in the world from Aug. 18, 2008 to June 2, 2009, from June 7, 2010 to June 20, 2011 and from Oct. 7 2013 to June 23, 2014. Meanwhile, Federer was World No. 1 from Feb. 2, 2004 to Aug. 11, 2008 and July 6, 2009 to May 24, 2010. Both Nadal and Federer were consistently among the top three or four players during the past decade.

Djokovic’s great 2015 season is similar to those of Nadal’s and Federer’s during their time at the top spot. The Serbian has won 11 titles in 2015 including Grand Slam wins at the US Open, Wimbledon Championships and Australian Open. Djokovic was World No. 1 from July 4, 2011 to June 25, 2012 and from Nov. 5, 2012 to Sept. 30, 2013. He is currently the World No. 1 holding the top spot since July 7, 2014.

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