Freddie Roach has warned Anthony Joshua not to trade punches with Wladimir Klitschko because of the risk he would be knocked out.

The respected American - widely considered the world's leading trainer - worked with Klitschko for four victories until being succeeded by the late Emanuel Steward.

Klitschko was then reinvented from a vulnerable fighter into both the dominant heavyweight of his era and one of the finest in history, and at Wembley on Saturday faces his defining fight in attempting to win the IBF and WBA titles.

The 27-year-old Watford-born fighter remains the favourite but the most widely-held opinions are that he will either stop Klitschko, 41, or be outboxed by the more polished, experienced former champion.

However while Roach, best known for his impressive work with Manny Pacquiao and presently in the UK to train Scott Quigg, is predicting a Joshua victory he is adamant if he attempts to exchange with Klitschko he will be stopped.

He even feels that Klitschko's best chance of victory comes in pursuing the knockout - that he is the more powerful puncher of the two - and that he should do so late in the fight instead of working for the points victory popular opinion would suggest.

"It's the age (that makes me think Joshua will win): he's 41-years-old now, he's been around for a long time," said Roach. "When I was training him he was a great puncher, and he either won by knockout or lost by knockout. But then Emanuel changed him into a good boxer.

"Youth is a great thing to have. But it's not enough alone. Joshua's got to be careful in exchanges, because if he gets caught up in exchanges, and they're both swinging, Klitschko does have knockout power. The left hook's always been his punch: he has a great left hook.

"He can't get caught swinging with this guy. If he trades punch for punch, Klitschko is the better puncher.

"Him and his brother (Vitali) are the hardest workers out there (in the heavyweight division). This was one of their workouts: running 12 x 800m sprints, under three minutes, with a minute's rest in between.

"That's an unbelievable workout for a flyweight, let alone a heavyweight, but he could do it. That's why they're so good, because they work their a**** off.

"Klitschko (can win by) taking him into the late rounds, get him to start exchanging late."

This weekend also brings Quigg's first fight since joining Roach at the Wildcard Gym in Los Angeles, when he faces Romania's Viorel Simion in an eliminator to challenge IBF featherweight champion Lee Selby.

The American revealed Quigg is the only fighter he has trained whose work-rate exceeds Pacquiao, and also that he has been focusing on training him to shut his mouth when he fights.

"I didn't think it was possible (to work harder than Pacquiao) but the son of a b**** is worse than Manny," he said. "I can't get him out of the gym.

"Whenever I'd try to get Manny to take a day off, it was like pulling teeth, I'd have to get his wife to help me to tell him that they had to go shopping together.

"I said 'Why did you break your jaw in your fight (against Carl Frampton)?' Because you f****** open your mouth too much. You say 'Who, who' when you punch: is that giving you any more energy, does it make the punch any harder?'

"Keep your mouth shut and you won't get a broken jaw any more."