Ask Rafael Nadal how his famously troublesome left knee is feeling on the eve of the US Open, and he baulks a bit while formulating a response.

“I am …” the 12-time major champion begins.

“You know …” he resumes, before smiling sheepishly and pausing again.

Eventually, Nadal offers something of a complicated answer, saying that he figures he needs to say he’s doing well because of how well he’s been playing.

He takes a 10-match winning streak into Monday against American Ryan Harrison as the year’s last Grand Slam tournament begins at Flushing Meadows.

The second-seeded Nadal says it “will sound strange” if he says he still has problems with his knee. But he adds he still has “pain some days”.