MONTREAL -- Spain's Rafael Nadal fears he

won't be fully fit for the U.S. Open despite making his return

to action in next week's Montreal Masters.

Nadal has been out of action for two months with knee

problems during which time he lost his French Open and

Wimbledon titles and world No. 1 ranking to Swiss rival

Roger Federer.

"I can't tell you if I'll be 100 percent for the U.S.

Open," Nadal told Reuters in an exclusive interview on Friday.

"It depends on many factors but clearly, I'll work as hard

as I can try to be in condition there. But most of all, I want

to make sure my knees respond well.