John Isner believes Andy Murray has what it takes to remain at the top of men's tennis for the foreseeable future.
Murray will officially overtake Novak Djokovic in the ATP world rankings on Monday and celebrated his rise to the top by beating Isner 6-3 6-7 (4-7) 6-4 in Sunday's Paris Masters final.
Having had a close-up of Murray in the French capital, American Isner says Murray can go from strength to strength, but warned the depth of quality means doing so will be no easy feat.
"I think he [Murray] does have that in him [to be number one for a long time]," he said. "But it will be tough.
"Novak sustained such an incredible level for so long, you know, and then Roger [Federer] before him. So we are in an era right now, it's pretty spectacular with these guys.
"Everyone keeps upping the ante. Right now it's Murray doing that. So he achieved his goal of getting to number one.
"I know he had that written down forever. But I think his next goal is staying there for a very long time. He's going to give everything he has to achieve that."
On his defeat to Murray, Isner felt that the scoreboard perhaps did not reflect how the deciding set went.
He added: "It was very close. I had a few opportunities, but really it's one set all.
"He was the better player in the third set. So, yeah, on the scoreboard it looks pretty close, but I guess it wasn't as close as it appeared.
"It's been a good week for me. It's definitely disappointing. I certainly wasn't happy [just] to be there today [Sunday], getting to the finals, I wasn't happy about that. I wanted to win, and I expected to win. I just fell a little bit short."