NEW YORK — Ask Rafael Nadal how his famously troublesome left knee is feeling on the eve of the US Open, and he’ll balk a bit while formulating a response.
‘‘I am . . . ’’ the 12-time major title winner began, haltingly, during an interview.
‘‘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 final Grand Slam tournament begins Monday, none of the players setting foot on the 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 US 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 12th-seeded Kirsten Flipkens of Belgium. At night, Williams’s younger sister Serena, the defending champ, plays 2010 French Open winner Francesca Schiavone of Italy, while 17-time major champion Roger Federer faces 62d-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 US 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.’’
Hard courts could exacerbate the matter because of the pounding legs take on the surface. Nadal maintains that there should be more tournaments played on other kinds of courts.
Yet he’s been successful everywhere, winning the Australian Open and US 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. ‘‘So I’m going to have to bring a really high level out.’’
Nadal’s rivals at the top of the game have taken note, too, of course.
Summed up defending champion Andy Murray: ‘‘He’s going to be very difficult to beat here.’’