Taylor Fritz celebrated his 24th birthday by beating doubles partner Tommy Paul 7-6 (3), 6-4 to advance to the quarterfinals of the St. Petersburg Open on Thursday.

It was a bittersweet victory because the Americans are good friends.

“It makes it even tougher - not just another American, one of my closest friends,” the fifth-seeded Fritz said in his on-court interview. “It’s never easy playing against a really close friend.”

The California native jumped out to a 3-0 lead in the first set and capitalized on Paul’s early double-fault in the tiebreaker. Paul converted just one of his four break points.

Fritz will next face Australian John Millman, who advanced by upsetting fourth-seeded Aslan Karatsev 6-3, 6-2. Karatsev won the Kremlin Cup on Sunday but the Russian lost his cool at 2-2 in the second set.

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He handed the 57th-ranked Millman a break when he was penalized a point for slamming his racket on the court after sailing a backhand wide at deuce.

The loss was also a big setback in Karatsev’s bid to qualify for the ATP Finals. The veteran Millman is 0-5 in quarterfinal matches this year.

In another upset, Dutch qualifier Botic van de Zandschulp set up a quarterfinal against home favorite and defending champion Andrey Rublev after beating eighth-seeded Sebastian Korda of the United States 6-2, 7-5.

Van de Zandschulp beat Rublev during the qualifying rounds in Rotterdam in February 2017, when the now sixth-ranked Rublev was still outside the top 100.

“These days it’s a little bit different. The tables are a little bit turned,” the 69th-ranked Van de Zandschulp said Thursday in his post-match press conference. “I’m eager to play tomorrow, and we will see.”

Van de Zandschulp, a U.S. Open quarterfinalist, broke the 38th-ranked Korda immediately and said he “didn’t give him free points,” limiting the American to two break chances.

Croatia’s Marin Cilic and Spain’s Roberto Bautista Agut will also meet in the quarterfinals Friday. Cilic beat sixth-seeded Karen Khachanov of Russia 6-4, 6-7 (1), 6-4 while the third-seeded Bautista Agut ousted American Mackenzie McDonald, 6-3, 6-2.