The night match capped the last of round two at the New York grand slam on another sweltering day that saw other top seeds advance and Serena and Venus Williams set up their 30th career meeting for Friday.
Top-seeded Nadal won 84 per cent of his first serves, made 82 per cent of his net points and mixed in lobs and drop shots to confound the unseeded 28-year-old Pospisil, ranked 88th.
The Spaniard got an early break in the third set and raced ahead to a 6-3, 6-4, 6-2, ending a match that had both players dripping in sweat. He'll face 27th-seed Karen Khachanov, 22, of Russia in the third round.
In other men's matches Andy Murray saved his best for last but still came up short in losing to Fernando Verdasco despite being the crowd favourite. The 34-year-old Spaniard sealed the 7-5, 2-6, 6-4, 6-4 win on his third match point to move on to the third round.
It was only the second time in 15 matches that Verdasco, seeded 31st, had defeated Murray, 31, the 2012 US Open champion, who has tried to mount a comeback after hip surgery earlier this year.
Verdasco will face Juan Martin del Potro in the third round. The 3rd-ranked Argentine defeated Denis Kudla of the United States 6-3, 6-1, 7-6 (7-4) in his second-round match.
Serena Williams, 36, served 13 aces in a 6-2, 6-2 defeat of Carina Witthoeft of Germany to set up the match against her sister. The 38-year-old Venus advanced earlier Wednesday with a straight-sets win over Camila Giorgi of Italy.
Serena, seeded 17th, has won 17 of the 29 matches the two have played in their careers dating back 20 years, but Venus, seeded 16th, won the last match in March at an event that marked Serena's return after maternity leave.
Defending champion Sloane Stephens also advanced after coming alive late to outlast qualifier Anhelina Kalinina and avoid an embarrassing shock. The third-seeded Stephens dropped the first set and barely held on in the second before emerging the winner 4-6, 7-5, 6-2.
The Ukrainian initially matched Stephens with a strong all-court game that had the champion scrambling for an answer.
"I wasn’t playing my best but I knew I had to find a way to make some plays," Stephens said. "It was hot and I was sweating a lot, but I came out in the third set bouncing around and ready to go. I never gave up, and I fought my tail off the whole time."
Stephens, 25, who lifted her first and so far only grand slam trophy in New York last year, advances to a third-round meeting against Victoria Azarenka who drubbed 25th-seeded Daria Gavrilova of Australia, 6-1, 6-2.
Stan Wawrinka, 33, had a sloppy win against French qualifier Ugo Humbert 7-6, 4-6, 6-3, 7-5. The Swiss, still working his way back after knee surgery, hit 57 unforced errors against the 20-year-old qualifier.
"I knew it was going to be a difficult match. He was a younger player, qualified," Wawrinka said, noting the tough conditions."
Tournament officials implemented an extreme heat policy, which allows players to take a 10-minute heat break between the second and third sets for women and the third and fourth sets for men.
Wimbledon runner-up Kevin Anderson of South Africa won his second-round match 6-2, 6-4, 6-4 over Jeremy Chardy, 31, of France. Anderson, the world number five, was the runner-up in the 2017 US Open.
Anderson, 32, is good friends with Chardy, but said he treated the match like any other.
"We know how each other plays a lot. There's no question about that," Anderson said. "But I wasn't really thinking I need to change something. You are always having to adapt."
Top-seeded Nadal won 84 per cent of his first serves, made 82 per cent of his net points and mixed in lobs and drop shots to confound the unseeded 28-year-old Pospisil, ranked 88th.
The Spaniard got an early break in the third set and raced ahead to a 6-3, 6-4, 6-2, ending a match that had both players dripping in sweat. He'll face 27th-seed Karen Khachanov, 22, of Russia in the third round.
In other men's matches Andy Murray saved his best for last but still came up short in losing to Fernando Verdasco despite being the crowd favourite. The 34-year-old Spaniard sealed the 7-5, 2-6, 6-4, 6-4 win on his third match point to move on to the third round.
It was only the second time in 15 matches that Verdasco, seeded 31st, had defeated Murray, 31, the 2012 US Open champion, who has tried to mount a comeback after hip surgery earlier this year.
Verdasco will face Juan Martin del Potro in the third round. The 3rd-ranked Argentine defeated Denis Kudla of the United States 6-3, 6-1, 7-6 (7-4) in his second-round match.
Serena Williams, 36, served 13 aces in a 6-2, 6-2 defeat of Carina Witthoeft of Germany to set up the match against her sister. The 38-year-old Venus advanced earlier Wednesday with a straight-sets win over Camila Giorgi of Italy.
Serena, seeded 17th, has won 17 of the 29 matches the two have played in their careers dating back 20 years, but Venus, seeded 16th, won the last match in March at an event that marked Serena's return after maternity leave.
Defending champion Sloane Stephens also advanced after coming alive late to outlast qualifier Anhelina Kalinina and avoid an embarrassing shock. The third-seeded Stephens dropped the first set and barely held on in the second before emerging the winner 4-6, 7-5, 6-2.
The Ukrainian initially matched Stephens with a strong all-court game that had the champion scrambling for an answer.
"I wasn’t playing my best but I knew I had to find a way to make some plays," Stephens said. "It was hot and I was sweating a lot, but I came out in the third set bouncing around and ready to go. I never gave up, and I fought my tail off the whole time."
Stephens, 25, who lifted her first and so far only grand slam trophy in New York last year, advances to a third-round meeting against Victoria Azarenka who drubbed 25th-seeded Daria Gavrilova of Australia, 6-1, 6-2.
Stan Wawrinka, 33, had a sloppy win against French qualifier Ugo Humbert 7-6, 4-6, 6-3, 7-5. The Swiss, still working his way back after knee surgery, hit 57 unforced errors against the 20-year-old qualifier.
"I knew it was going to be a difficult match. He was a younger player, qualified," Wawrinka said, noting the tough conditions."
Tournament officials implemented an extreme heat policy, which allows players to take a 10-minute heat break between the second and third sets for women and the third and fourth sets for men.
Wimbledon runner-up Kevin Anderson of South Africa won his second-round match 6-2, 6-4, 6-4 over Jeremy Chardy, 31, of France. Anderson, the world number five, was the runner-up in the 2017 US Open.
Anderson, 32, is good friends with Chardy, but said he treated the match like any other.
"We know how each other plays a lot. There's no question about that," Anderson said. "But I wasn't really thinking I need to change something. You are always having to adapt."