Andy Murray says Novak Djokovic is set to dominate men's tennis for years to come, with a younger generation of players still well below his level.
The Serb, 36, won a record-equalling 24th Grand Slam singles title with victory over Daniil Medvedev in the US Open final in New York on Sunday.
He has no immediate retirement plans.
"It's up to the young guys to be pushing Novak and looking to overtake him. It doesn't look like that's close to happening," Murray told BBC Sport.
"Novak's been an incredible player for a very long time, like Roger and Rafa, it's the longevity and how long they've all been able to do that for.
"Novak's longevity has been the greatest. He's played at this level for such a long time now."
Djokovic was beaten in this year's Wimbledon final by 20-year-old Spaniard Carlos Alcaraz.
Three-time Grand Slam winner Murray, who was speaking on the eve of Great Britain's Davis Cup campaign, said the wrong conclusions were drawn from Djokovic's defeat at SW19.
Murray, 36, added: "I said that after Wimbledon, everyone was saying it was a changing of the guard, but it wasn't for me.
"It was quite clear Novak had won two of the first three slams and was very, very close in the final of Wimbledon.
"He proved that he's the best player in the world again yesterday and like I said it's up to the youngsters to improve enough to take him.
"I don't think he's going to stop at 24 titles, but you never know how long it will last. Physically, motivation, whether someone like Alcaraz improves - he's an amazing player Alcaraz, he's brilliant, but to do what Novak is doing on a consistent basis is different.
"It's not the same as doing it for 15-20 weeks a year like Novak is doing, every time he steps on the court. I do think that Alcaraz will soon get to that level but I don't think this is going to be the last time that Novak wins a Grand Slam."
How to watch the Davis Cup
All of Great Britain's matches will be streamed on BBC iPlayer and the BBC Sport website from 13:00 BST with updates on BBC Radio 5 Live.