Kostya Tszyu and his trainer Johnny Lewis believe his mental toughness will give him an edge in his world boxing title defence.

Tszyu, Lewis and five other members of the fighter's team left Sydney on Sunday for the United States where the 35-year-old makes his long-awaited return to the ring against American Sharmba Mitchell in two weeks.

A professional for 13 years, Tszyu admitted to butterflies in his stomach as he prepares to return to the ring after the 22-month lay-off caused by serious Achilles tendon and shoulder injuries.

Tszyu said he was looking forward to proving he's "the best in the business" in the International Boxing Federation super lightweight title defence.

"When I win it's going to be probably the best victory (of my career)," Tszyu said at Sydney Airport.

"Because of those two years off (from) the ring, those have been hard times with injuries, it wasn't an easy journey.

"But it's only two weeks away and I'm expecting a great, great performance from myself."

Tszyu said his preparation had entailed "a lot of pain and sacrifice" and he had several reasons why he'd have an advantage.

"One of them (reasons) is my desire and my will to win," Tszyu said.

"The amount of time I've spent in the gym just preparing for this fight is a lot. It means I just cannot lose, I just can't think this way, it's just impossible for me."

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