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 number one ranking to Swiss rival Roger Federer.
"I can't tell you if I'll be 100 percent for the US Open," Nadal said 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.
"Once I know my knees will respond well, I can train well, I can compete with greater calm and that's what will give me, little by little, the confidence to train at the maximum level," he said.
The US Open, the only Grand Slam that Nadal has yet to win, starts on August 31 at Flushing Meadows, New York.
The Spaniard has been suffering from tendonitis in both knees and has not played a competitive match since being knocked out in the fourth round of the French Open by Swede Robin Soderling on May 31.
Nadal pulled out of Wimbledon just three days before the tournament was due to begin but said he is now ready to begin the fight back to his best form.
"I arrived at two very important tournaments this season, Roland Garros [French Open] and Wimbledon without my best condition but for the rest I am here and I am very happy to come back on court with the best motivation to work hard to try to play my best tennis as soon as possible.
"I know its going to be tough in the beginning because after two months outside of competition its always tough to come back after an injury but I am going to be ready to work very hard as soon as possible," he added.
CONGESTED CALENDAR
Nadal has blamed his injuries on the congested calendar for leading players but said it would be a tough process to reform the schedule.
"Well, it's clear that the calendar can't be perfect for everyone but I think that we -- the players, the ATP -- are working hard to try to develop the best possible calendar for the players and for the tournaments.
"I think that everyone knows that starting on January 1 and ending on December 5 is too long of a calendar but it's not easy to fix because there are many tournament interests at play and everything is very difficult. We can't scorn any tournament.
"We must try to find the best possible solution that more or less pleases everyone," he said.
Nadal could face the ultimate fitness and mental test in Montreal, with world number one Federer confirming on Friday that he would compete in the event.
Photograph: Reuters