Roger Federer and Rafael Nadal have been warned by Marat Safin that age will catch up with them.
Federer, 36, won his 18th and 19th Grand Slams this season by taking the Australian Open and Wimbledon titles.
The French Open and US Open went to Nadal, 31, as the pair continued their dominance of the sport.
But legend Safin believes it will only get harder for them to keep bringing home the silverware.
He told the Independent: "Federer and Nadal are great players but they're getting older. No matter how much you work in the gym, it becomes harder and harder to recover match after match. Age catches up with you."
Safin was in London this week to compete at the Champions Tennis legends tournament. While there, he delivered a brutal assessment of the next generation of the sport's stars.
Nick Kyrgios hasn't hit the heights this season
The Russian is a big fan of Nick Kyrgios - the pair both have fiery attitudes - but Safin is expecting to see more in terms of results.
He said: "If Federer and Nadal are still winning I think there's something wrong. I don't see any upcoming superstars today."
"I'm not saying that our times were the best, but when I was growing up, players were winning ATP tournaments at 16, 17, 18.
"Now players are only just starting to be pros at the age of 25. I don't know why that is.
Rafael Nadal and Roger Federer won all four Grand Slams between them in 2017
"Players used to retire by the time they got to 30. At 32 you were a dinosaur. Now you see players who are still running at the age of 38.
"The upcoming young guys just aren't at a high enough level. If you can still manage to run at the age of 38 and still be No 1 in the world, it means there must be something wrong with the other players.
"If you want to be a really good pro you need to be beating Nadal and Federer now/
"Look at Murray and Djokovic. They were beating the top players when they were 19 or 20, but you just don't see that from the younger players today."