NEW YORK, Sept 5: Juan Martin del Potro’s stay at the U.S. Open really should be over. Nearly was.

The 2009 champion at Flushing Meadows had a fever and thought about quitting while dropping the opening two sets with little resistance against No. 6-seeded Dominic Thiem in the fourth round Monday. Then he trailed by a big margin in the fourth set, even facing two match points.

Still, buoyed by a boisterous Grandstand crowd chanting “Ole!” and his nickname, “delPo,” he never gave in or gave up. Eventually, del Potro worked his way all the way back on the strength of powerful serves and thunderous forehands to edge Thiem 1-6, 2-6, 6-1, 7-6 (1), 6-4 and set up a quarterfinal showdown against Roger Federer.

“I took all that energy to change in a good way and think about (fighting) and not (retiring),” the 24th-seeded Argentine said. “Unbelievable atmosphere.”

Del Potro long has been popular, but became even more so since returning from three operations on his left wrist that kept him out of Grand Slam action for two years and sent his ranking out of the top 100.

“He was gone for so long that it’s just really nice to see him back playing these kind of matches,” Federer said. “That’s what he came back for, to get crowd support the way he got it.”

Thiem also aided del Potro by playing his worst tennis when he was closest to victory in the fourth set, which he led 5-2. Thiem served for the match at 5-3, but got broken. Leading 6-5, he managed to get within a point of winning at 15-40 on del Potro’s serve, but a pair of aces at 127 mph and 121 mph erased those two chances. The ensuing tiebreaker was dominated by del Potro, who closed it with a booming cross-court forehand winner on the run.

In the fifth set, del Potro closed things on his second match point, when Thiem double-faulted. How close was this? Thiem actually won more points, 141-139.

When it was over, del Potro raised both arms overhead and threw his head back , enjoying the fans’ adulation, then crossed himself. He joked that he thought he should get a trophy just for winning this one.

It was by far the day’s most enthralling match, with spectators’ roars heard all the way across the grounds at Arthur Ashe Stadium, where Federer was beating No. 33 Philipp Kohlschreiber 6-4, 6-2, 7-5.

The only bit of intrigue came after the second set, when Federer went off the court for a medical timeout. He said afterward it was so he could get a massage after feeling a muscle tightening at the top of the back of his left leg.

His back had been bothering Federer before the U.S. 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.

Federer improved to 12-0 against Kohlschreiber; his record against del Potro is 16-5. But del Potro won their meeting in the 2009 final in New York in five sets for his only Grand Slam title, ending Federer’s streak of five straight U.S. Open championships — and he hasn’t won the trophy since.

“I felt like that I left that match with a lot of regrets,” Federer said.

The other matchup on that half of the men’s bracket will be No. 1 Rafael Nadal against 19-year-old Andrey Rublev, the youngest quarterfinalist at the U.S. Open since Andy Roddick was 19 in 2001.

Nadal got to the round of eight in New York for the first time since his 2013 title, overwhelming 64th-ranked Alexandr Dolgopolov 6-2, 6-4, 6-1.

“Every victory, every set that you win, is more confidence,” Nadal said. “That what’s I am doing.”

Now Nadal gets to play a fifth consecutive unseeded foe in Rublev, who took out No. 9 David Goffin 7-5, 7-6 (5), 6-3.

There are four Americans in the women’s quarterfinals in New York for the first time in 15 years after No. 15 Madison Keys grabbed the last four games to top No. 4 seed Elina Svitolina of Ukraine 7-6 (2), 1-6, 6-4 in Monday’s last match.

No. 20 CoCo Vandeweghe beat Lucie Safarova 6-4, 7-6 (2) earlier Monday, while Venus Williams and Sloane Stephens advanced Sunday.

Vandeweghe’s quarterfinal opponent is No. 1 seed Karolina Pliskova, the 2016 runner-up, who beat 91st-ranked American Jennifer Brady 6-1, 6-0.

Keys now faces Estonia’s Kaia Kanepi, who beat Daria Kasatkina 6-4, 6-4 to become the first qualifier in 36 years to reach the U.S. Open women’s quarterfinals.

The 32-year-old Estonian is ranked 418th after missing much of the past two years with problems in the soles of both feet and a virus known as a precursor to mononucleosis.

“It’s pretty amazing where I am now,” Kanepi said, “compared to where I was few months ago.” AP