Nadal led 6-3 3-4 in Monday's late tie on Arthur Ashe Stadium before Ferrer had to give up with a calf injury.

Ferrer, the 36-year-old former world number three, will retire after playing in Barcelona or Madrid next year, but says this will be his final Grand Slam.

"It is sad for me to see him finish like this," said 32-year-old Nadal.

"I'm very very sorry for him, he is one of my closest friends on tour and we have shared amazing moments together, playing in French Open finals and playing together in the Davis Cup and Davis Cup finals.

"He is one of the greatest players we have had in our country."

Top seed Nadal is among the favourites to retain his crown and has a seemingly favourable draw at Flushing Meadows, with two top 10 players - Kevin Anderson and Dominic Thiem - in his quarter.

Anderson lost to Nadal in last year's final and has never beaten him in five attempts, while Thiem has only won four matches since finishing runner-up to the Spaniard at the French Open in June.

Before looking that far ahead, Nadal must focus on beating Canada's Vasek Pospisil, ranked 88th, in the second round on Wednesday.