Ferrer, whose decision to retire brings the curtain down on his final appearance at a Grand Slam, said he had taken an injection before the match but was in obvious discomfort and need treatment courtside before he chose to quit.
"I'm improving tournament after tournament, match after match".
Nadal's potential semifinal opponent, Argentine third seed and 2009 champion Juan Martin del Potro, reached the second round with a 6-0, 6-3, 6-4 win over USA qualifier Donald Young.
The Brit leads head to head meetings 13-2, Verdasco only beat him at 2003 Segovia Challenger and 2009 Australian Open.
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".
Serena Williams made a triumphant return, opening her bid for a record-tying 24th Grand Slam title with a 6-4, 6-0 first-round victory over Magda Linette.
Halep was beaten 6-2, 6-4 by 44th-ranked Kaia Kanepi of Estonia on Monday in a match that was stunningly lopsided and lasted all of 76 minutes.
Djokovic arrived in NY as many people's favourite for the trophy after beating Roger Federer to become the first man to win all the nine different Masters titles in Cincinnati last weekend.
Twice a Grand Slam semifinalist, Johanna Konta can't seem to make any headway at the majors these days.
Russian Federation to hold biggest war games since Cold War
China's and Mongolia's armed forces will provide ground troops as well as planes, helicopters and command posts, the source added. Since Moscow annexed Ukraine's Crimea in 2014, has increased forces in eastern Europe to deter more Russian military action.
Ferrer was asked whether he regretted playing at a time when the Big Four of Federer, Nadal, Djokovic and Murray dominated the sport.
"I've lost a lot of matches out here over the years, I've struggled a lot, but it's like a new beginning".
As for the players who have to cope with the sweltering conditions, it is all part of the brutal test that is the fourth and final Grand Slam of the year. "But being honest, I am sure he will have the goodbye that he deserves on the events that he decides to play".
Shapovalov was leading 7-5, 5-7, 4-1 when Auger-Aliassime retired.
"It's even a bigger priority this year, the US Open, than it has been last year".
The 44th-ranked Tiafoe had lost in the first round in NY in each of the past three years, including against Roger Federer in 2017.
Wawrinka, the former world No. 3, now down at 101, shrugged off having to take a medical time-out early in the third set.
It was the first match at the rebuilt Louis Armstrong Stadium. Nadal and Ferrer, who contested the 2013 Roland Garros final, became the fifth pair in the Open Era to meet in the opening round of a major as well as a championship match.
In one of the more intriguing matches, Canadian teens Denis Shapovalov and Felix Auger-Aliassime clashed for a first time and Shapovalov took via the worst way, the Auger-Aliassime in tears after retiring down 7-5, 5-7, 4-1 due to an irregular heartbeat.