NEW YORK — Rafael Nadal easily returned to the US Open quarterfinals for the first time in four years by routing Alexandr Dolgopolov, 6-2, 6-4, 6-1, Monday.
Nadal turned in his most powerful performance yet on a dominant day for the No. 1 seeds, and moved a win away from a potential semifinal showdown with longtime rival Roger Federer.
Federer elminated No. 33 Philipp Kohlschreiber, 6-4, 6-2, 7-5, Monday night, the only bit of intrigue coming after the second set, when Federer left to take a medical timeout.
Juan Martin del Potro, looking sick and sluggish as can be at the outset, staved off sixth-seeded Dominic Thiem on the strength of powerful serves and thunderous forehands for a 1-6, 2-6, 6-1, 7-6 (1), 6-4 victory over more than 3½ hours.
It was by far the day’s most enthralling match, with spectators’ roars heard all the way across the grounds at Arthur Ashe Stadium.
Del Potro set up a quarterfinal showdown against Federer.
Federer’s back had been bothering him before the US Open and restricted his practice time, something he blamed for problems while getting pushed to five sets in each of the first two rounds last week. But the lopsided win against Kohlschreiber — who never held a break point — was Federer’s second in a row in straight sets.
Nadal punished an opponent who had beaten him in two of the previous three meetings shortly after top-seeded Karolina Pliskova overwhelmed American Jennifer Brady, 6-1, 6-0, in just 47 minutes,
‘‘Every victory, every set that you win is more confidence. That what’s I am doing,’’ Nadal said.
He never gave the unseeded Dolgopolov a chance, breaking serve six times in the 1-hour-41-minute match.
Nadal hadn’t reached the quarterfinals in Flushing Meadows since the second of his two US Open titles in 2013.
He advanced to face 19-year-old Russian Andrey Rublev, the youngest quarterfinalist since Andy Roddick had just turned 19 in 2001.
Rublev upset No. 9 seed David Goffin, 7-5, 7-6 (7-5), 6-3, and will be Nadal’s fifth straight unseeded opponent.
Nadal had dropped the first set in the two previous rounds but he was locked in from the start of his 50th US Open victory, never losing serve and facing just two break points.
Dolgopolov, who has been interviewed by anti-corruption investigators about a match with unusual betting patterns last month at another tournament, was hesitant to give Nadal too much credit.
‘‘I played bad,’’ Dolgopolov said. ‘‘He didn’t do much today.”
Kaia Kanepi beat Russia’s Daria Kasatkina, 6-4, 6-4, to become the first qualifier in 36 years to reach the US Open women’s quarterfinals.