Twenty-time Grand Slam champion Roger Federer is clearly still in love with a sport he still dominates at 36 years of age, and he has expressed bafflement at claims by Andre Agassi in his searingly honest autobiography 'Open' that he at times hated a game which brought him such fame and fortune.

Agassi, in a remarkable and fluctuating career, won eight Grand Slams, but shockingly revealed in his book that he didn't always enjoy the game.

He wrote: "I play tennis for a living even though I hate tennis, hate it with a dark and secret passion and always have."

Such words are incomprehensible to a man like Federer, who is on the cusp of becoming the oldest world number one in the open era and he recently doubted the validity of Agassi's claims.

“Agassi said the same after he retired and he played for 20-something years," said Federer.

“I don’t believe him as well, everything that he said in that book."

The Swiss ace was speaking in defence of under fire Aussie duo Nick Kyrgios and Bernard Tomic, who have received stinging criticism for their attitude during their turbulent careers.

Both touted in the past as players capable of rising to the top of the sport, they have been accused of apathy and arrogance as they have so far failed to come near fulfilling their potential.

Tomic, who has slumped to 169 in the world, has regularly been accused of 'tanking' matches and recently dismissed reporters after a recent loss by saying that he was going to have fun counting his money.

Kyrgios has a healthier ranking of 14, but still infuriates fans with his petulant demeanour and inconsistent displays. 

Federer, however, maintains his faith in the duo and insists that they are not lacking in motivation.

"I believe they [Kyrgios and Tomic] do enjoy their time on the courts sometimes," Federer said, as quoted by Tennis World USA.

"And there are moments when they do win a match, or there are moments in practice where they actually do enjoy it. Maybe there are moments when things are difficult. When you're packing up the bags and travelling to the next event, or whatever it is," the Swiss ace explained.

"Then it becomes tricky sometimes. But that's the case for everybody. But it's also a question of seeing the glass half-full rather than empty. They still have some potential to fulfil. They know that and it's a question up to them if they want to do it or not."