Placing Nick Kyrgios in his top three players of the future, tennis superstar Roger Federer knows a changing of the guard is inevitable.
Federer has managed to keep generation next at bay so far at the ATP Finals in London, seeing off American upstart Jack Sock and then German prodigy Alexander Zverev in his first two pool matches to safely progress to a record 14th season-ending championship semi-final.
His three-set victory over Zverev also sent the 19-time grand slam champion soaring above Tiger Woods as sport's all-time highest prize money earner with a mind-boggling $US110,340,682 ($A145,180,678).
But, as evidenced by 12 of the top 20 players suffering serious injuries in 2017 and all of his big-four rivals - Rafael Nadal, Novak Djokovic, Andy Murray and Stan Wawrinka - calling a premature end to their seasons, it's not getting any easier for the 36-year-old father of four.
Unlike most, the pragmatic Swiss refuses to blame brutal hard courts or the gruelling length of the season for their demise.
"Shave 10 years off our age and we probably will do better," Federer said.
"A lot of the guys are just touching 30-plus, you know. Back in the day, at 30, a lot of guys were retiring - Edberg, Sampras.
"It was like normal at 29, 32, to start looking towards the end of your career. Now you guys expect everybody to play till 36.
"When somebody is injured at 31, it's like, 'Oh my God, how is this possible?'
"Actually, it's a normal thing."
Federer, enjoying an incredible renaissance after spending the last six months of 2016 rehabilitating from knee surgery, says lengthy breaks from the game were the only way to extend a career.
Interestingly, he also believes double-handed backhands can be crippling.
"(A) wrist (injury) to me is not something that comes from overplaying, especially if it's your left hand," he said.
"That's just a move I can't relate to because I don't play double-handed. From what I've been told, the wrist is always in a troubled position with that flicking that's going on more and more these days.
"I think Andy (Murray) even broke his wrist. Maybe that was something else, I'm not sure. Del Potro has had three of them. I think these things tend to happen."
After losing to him in Madrid two years ago, Federer enjoyed two titanic tussles with Kyrgios this season in Miami and Prague and views the Australian alongside Zverev and Canadian teenager Denis Shapovalov as the heirs apparent to the big five.
But he suspects Zverev is leading the way.
"I do see a great upside from all these three guys. What I like about Zverev is he's got the full package," Federer said.
"He's already three in the world. Then, of course, he's only going to get stronger from here."
