Defending champion Rafael Nadal reached the second round of the US Open in unfortunate circumstances with opponent David Ferrer forced to retire during his final grand slam match.

The 36-year-old, one of the outstanding players of this era, is renowned as a warrior and had never pulled the plug midway through a contest in 207 previous grand slam matches.

He was a break up on Nadal at 4-3 in the second set having lost the opener 6-3 in steamy conditions on Arthur Ashe Stadium but was clearly struggling with a left calf injury and decided he could no longer continue.

Ferrer plans to retire at an event in Spain next season, and he told the crowd: "It was pain. I tried to play but I think it (his calf muscle) is broken. I have really good memories here. This is my last grand slam. I'm so sorry because I can't finish the match. I will miss you a lot."

After a fine start to the match, Nadal's forehand went awry in the second set, but his primary feeling at the end of the clash was empathy for his compatriot.

The world number one said: "I'm very, very sorry for him. He's one of my closest friends on tour. We shared amazing moments together playing French Open finals, a couple of Davis Cup finals. It's sad to see him finish like this but he deserves everything because he's a fantastic player."

Ferrer reached his only slam final at Roland Garros in 2013 and climbed as high as world number three the same year.

Nadal moves through to a second-round clash with Canada's Vasek Pospisil while Stan Wawrinka is also on an eight-match winning streak at Flushing Meadows after defeating Grigor Dimitrov for the second successive slam.

The Swiss won his third slam title here two years ago but was unable to defend it after undergoing knee surgery from which he has struggled to recover.

Having returned to the tour at the Australian Open, it is only in the last month or two that Wawrinka has started to look anything like his old self, a sequence that began with a shock victory over Dimitrov at Wimbledon.

Their respective form since them made this far less of an upset, although Dimitrov was the eighth seed while Wawrinka is still ranked just outside the top 100.

After his 6-3 6-2 7-5 victory, the 33-year-old said: "There is a lot of question marks of how my body will be right, how mentally I will be right. A few weeks ago I was still struggling a lot.

"I'm improving tournament after tournament, match after match. And I can see that the last tournaments, it went really high, from struggling in the match to competing at a really high level. So I'm really happy with that."

One of the most eagerly-anticipated matches of the day was between Canadian teenagers Denis Shapovalov and Felix Auger-Aliassime.

Shapovalov, 19, burst on to the scene last summer but for 18-year-old Auger-Aliassime this was his first taste of the big time after coming through qualifying.

Sadly for the younger man, he was forced to retire in tears in the third set after experiencing heart palpitations in the hot and humid conditions.

The pair are best friends, and Shapovalov consoled his compatriot before saying: "It's actually really tough to see him going out like this. I told him at the net we're going to be back here, we're going to play so many of these."

Man of the moment Stefanos Tsitsipas won his first main-draw match at Flushing Meadows, beating veteran qualifier Tommy Robredo 6-3 7-6 (7/1) 6-4, while there was an emphatic 6-0 6-3 6-4 victory over Donald Young for third seed Juan Martin del Potro.

Last year's runner-up Kevin Anderson was in deep trouble at two sets to one down to American Ryan Harrison and struggling with cramp but recovered to win 7-6 (7/4) 5-7 4-6 6-3 6-4.