Former world number one Boris Becker has called on tennis players under the age of 28 to "show up" and start challenging for Grand Slam titles.
With Nadal winning the French Open, the last 10 Slams have been won by either the 33-year-old, Novak Djokovic, 32, or Roger Federer, 37.
Dominic Thiem, 25, is the only male player currently aged under 28 to have reached a Grand Slam final.
"That is not good," six-time Grand Slam champion Becker said.
"That is not a compliment for anybody under 28.
"And don't give me that the others are too good. We should question the quality and the attitude of everybody under 28. It just doesn't make sense. As much as I respect Roger, Rafa, Novak - who else? Show up. Give me something I want to talk about."
Austrian Thiem has reached two successive French Open finals and took a set from Nadal at Roland Garros on Sunday.
He defeated Djokovic in the semi-finals, while Greek 20-year-old Stefanos Tsitsipas beat 20-time Grand Slam champion Federer at the Australian Open.
To date, Andy Murray, Stan Wawrinka, Marin Cilic and Juan Martin Del Potro - all now over the age of 30 - are the only players other than Nadal, Djokovic and Federer to have won a Grand Slam title since the Spaniard won at Roland Garros in 2005.
"Eventually they will be too old. But you want to see the passing of the torch while they are still in their prime," added Becker, who won Wimbledon at the age of 17.
"You want to see Stefanos and Dominic beating them when they are still very, very good.
"It's not the forehands. It's not the fitness. It's a certain mentality, mindset, attitude that makes the difference between winning and losing."
Nadal, Djokovic and Federer have won 53 Grand Slam titles between them, and there have long been fears as to what will happen to tennis' popularity once they retire.
"There will be a dip. People have to get used to the new players," said Becker. "There will be a 'wow' moment. 'Wow, we're not talking about Roger and Rafa any more'.
"But we said the same thing with [John] McEnroe and [Jimmy] Connors, [Andre] Agassi and [Pete] Sampras. Maybe myself and [Stefan] Edberg and [Mats] Wilander.
"Tennis will always continue with great new stars. But there will be a dip and then the spotlight will be on the young generation to say, 'Now, show up. Who are you? Are you good enough, can you carry the sport, or was it all a bluff?'"