The world's leading trainer believes Amir Khan can succeed in upsetting Terence Crawford — even though he considers the American the finest fighter in the world.

Khan challenges the great WBO welterweight champion at New York's Madison Square Garden on Saturday in what could prove to be his defining fight.

Trainer Freddie Roach is as familiar with Khan, having once trained him at his famous Wild Card gym, as he is with the level Crawford is competing at after inspiring the great Manny Pacquiao to his peak years.

Fights with Pacquiao and Floyd Mayweather previously eluded Khan when he hoped to challenge one of the world's two leading fighters, but this weekend the opportunity the 32-year-old from Bolton has desperately wanted finally arrives.

"Khan can pull off the upset if he fights the right fight," Roach told Press Association Sport.

"I always said, if Khan goes out there and boxes and wins one round at a time, and keeps doing it, and not look for the knock-out (he can win).

"When he hurts him, don't open up on him because that's when he gets caught, and puts himself in harm's way.

"He's a very good boxer; he still has a lot left in him. His speed; his foot-speed and hand-speed are unbelievable. I remember him and Pacquiao sparring competitively, it was about even. He's definitely at that high level; hopefully Father Time hasn't got to him yet.

"I still think he's young enough and fresh enough to have moments of success, but I like Crawford to win the fight.

"He's a big underdog; Crawford's the best fighter in the world. Pretty soon him and (fellow welterweight) Errol Spence are going to have to fight each other."

The decorated Roach is reminded of Mayweather-Pacquiao by the rivalry between Vasyl Lomachenko and Crawford, and also believes that Khan's task has been made even more difficult because Crawford will want to respond to Lomachenko's destruction of Anthony Crolla last weekend.

"He does have to produce a good performance," he said. "I've seen him go out and box before, he's a very good boxer, but I've seen him blow guys away also.

"He can start very quickly and knock out guys in only two rounds. He can punch, too, he's not just a good boxer but a good puncher also. He's more of a complete fighter than Lomachenko, and for what happened last week he's going to have to jump on Amir right away.

"You have to control your fighter and get him ready for what he's getting ready for, and make sure he stays on that (instead of being concerned with a rivalry). It pretty much affects everyone in camp."

:: Amir Khan fights Terence Crawford live on BT Sport Box Office, Saturday 20th April. For more information www.bt.com/btsportboxoffice