NEW YORK -- Ask Rafael Nadal how his famously troublesome left knee is feeling on the eve of the U.S. Open, and he'll balk a bit while formulating a response.

"I am ..." the 12-time major title winner began, haltingly, during an interview with The Associated Press.

"You know ..." he resumed, before smiling sheepishly and pausing again.

Eventually, Nadal offered something of a complicated answer.

"I have to say that I am very well, because the results have been amazing since I came back," he said. "If I say something else, (it) will sound strange."

That's because when the year's last Grand Slam tournament begins Monday, none of the players setting foot on the blue hard courts of Flushing Meadows possesses as much momentum -- or is in as fine form -- as the No. 2-seeded Nadal. He's won his past 10 matches heading into the first round against the 97th-ranked Ryan Harrison of the United States.

Another past U.S. Open champion on Monday afternoon's schedule is 2000-01 winner Venus Williams, a former No. 1 now ranked 60th who will be taking on recent Wimbledon semifinalist and 12th-seeded Kirsten Flipkens of Belgium. At night, Williams' younger sister Serena, the defending champion, plays 2010 French Open winner Francesca Schiavone of Italy, while 17-time major champion Roger Federer faces 62nd-ranked Grega Zemlja of Slovenia.

The biggest curiosity when it comes to Nadal these days is how his knees will hold up. They've presented recurring problems for him over the years, particularly the left one, which kept him out of action from late June 2012 until February 2013. He missed the London Olympics, last year's U.S. Open and this year's Australian Open.

"I feel more comfortable now than six months ago, that's for sure," Nadal said, then quickly added: "But I still have pain some days."

He was asked whether he thinks that might be the case for the rest of his career.

"Hopefully not," the 27-year-old Nadal said. "Hopefully not."

Hard courts could exacerbate the matter because of the pounding legs take on the unforgiving surface. Nadal himself maintains that there should be more tournaments played on other kinds of courts.

Yet he's been successful everywhere, winning the Australian Open and U.S. Open once each, along with Wimbledon twice, to go along with his record eight championships on the slower red clay of the French Open. Nadal is 15-0 on hard courts in 2013, with his current run of victories built en route to titles at Montreal and Cincinnati.

"He's on a great streak right now. He's playing fantastic tennis. He's playing as well as anyone in the world right now," said the 21-year-old Harrison, who will be making his Arthur Ashe Stadium debut against the tournament's 2010 champion.