PARIS — Second-seeded Alexander Zverev of Germany reached his first Grand Slam quarterfinal the hard way Sunday, beating Karen Khachanov 4-6, 7-6 (4), 2-6, 6-3, 6-3 for his third straight comeback win in five sets at the tournament.

The gritty victory came with the usual temper flare-ups from Zverev. But he also showed again that he knows how to dig deep when it matters. In all three of his five-setters he has trailed 2-1 in sets.

“I’m young. I might as well stay on court and entertain you guys,” the 21-year-old Zverev joked with the crowd. “This definitely paid off, the hours in the gym every day … Everything comes together slowly and I’m happy to be here.”

Zverev said he spends up to four hours each day working on weights and fitness.

He might need to up that schedule considering his quarterfinal opponent is No. 7 Dominic Thiem of Austria, who has reached the past two French semifinals and is one of the fittest players on the men’s circuit.

“I’m expecting another five-set match. I’ll get myself ready,” Zverev said. “I’m very happy about being in the quarterfinals here, going the hard way, going the long distance … showing everybody that I can play for as long as I need to.”

Thiem, who has played three straight four-setters, beat No. 19 Kei Nishikori of Japan, 6-2, 6-0, 5-7, 6-4

Nishikori won 14 points in the first set and nine in the second, losing to Thiem for the first time.

Novak Djokovic equaled Roger Federer’s mark of reaching nine consecutive quarterfinals by beating No. 30 seed Fernando Verdasco, 6-3, 6-4, 6-2. It’s also Djokovic’s 12th career trip to the quarterfinals in Paris, a record for the 50-year professional era. Djokovic advanced to his 40th Grand Slam quarterfinal.

Thiem, 24, leads 4-2 overall against Zverev but lost to him last month in the Madrid Masters final. Thiem called their upcoming quarterfinal “the matchup most fans in Germany and Austria were hoping for when they saw the draw.”

Zverev’s smash at the net gave him match point, which he took when Khachanov netted a forehand. Zverev sank to his knees in celebration. He then patted Khachanov, a friend from their junior days, on the chest before pumping his arms to celebrate.

As in the previous two rounds, Zverev struggled for consistency, illustrated in the 10th game of the second set.

Zverev was serving for the set at 5-4 but soon trailed 0-40.

Then he won the next four points to give himself a set point – only to miss a smash from near the back of the court. An ace gave him another set point but he also missed that one, putting a low volley into the net.

On his third set point, Zverev found the net again with a sloppy backhand. Khachanov punished him and broke for 5-5.

In women’s play, 13th-seeded Madison Keys reached the quarterfinals for the first time at the tournament by beating No. 31 Mihaela Buzarnescu of Romania, 6-1, 6-4.

“I made the final of the U.S. Open but lost to my friend Sloane Stephens,” Keys said. “Hopefully big things can happen here.”

Keys next faces unseeded Yulia Putintseva of Kazakhstan, who beat 26th-seeded Czech Barbora Strycova, 6-4, 6-3.

If she wins that match, Keys could meet Stephens in the semifinals.

Stephens, the 10th-seeded American, also reached the last eight in Paris for the first time, beating No. 25 Anett Kontaveit of Estonia, 6-2, 6-0.

Stephens next plays either Australian Open champion Caroline Wozniacki – seeded second – or 14th-seeded Russian Daria Kasatkina. Their match was suspended at 3-3 in the second set. Kasatkina win the first, 7-6.

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