Miami Open 2018: Roger Federer cannot beat John Isner when he does THIS - Greg Rusedski

Rusedski was particularly impressed with Isner’s serving against Marin Cilic in the round of 16 stage on Tuesday.

He beat the Croat in straight sets and recorded first-serve percentages of over 70 per cent to reach the quarter-final.

Isner is coming off the back of a victorious Indian Wells campaign, where he and fellow American Jack Sock beat the Bryan brothers in the men’s doubles final.

And Rusedski believes the likes of Roger Federer and Novak Djokovic would not be able to beat Isner on this form.

“He won the doubles title last week in Indian Wells which was big for him because he got some match play there,” Rusedski said on Sky Sports.

“He served unbelievably well in that first set (against Cilic). I mean it was blustery blowy, 77 per cent first serve in those conditions was impressive.

“On top of that you look at the overall for the match, 73 per cent.

“It’s going to be hard to beat John Isner with those sorts of statistics, I don’t care if you’re Federer, Djokovic or anybody.

“The other thing I liked about it he was more aggressive, he was going back behind Cilic a lot as well as willing to come forward.

“I like the way he played, he played like a big man.

“When he loses matches and doesn’t play well, usually he is too defensive and too tentative, he doesn’t go after it. Yesterday he did."