ST. PETERSBURG (Russia) -- Taylor Fritz celebrated his 24th Birthday by defeating Tommy Paul, his doubles partner, 7-6 (3), 6-4 in order to reach the quarterfinals at the St. Petersburg Open on Wednesday.

It was bittersweet because Americans are good friends.

Fritz, fifth-seeded, said that it made it more difficult -- not only another American but also one of my closest friends. "It's not easy to play against a close friend.

The native of California jumped to a 3-0 lead during the first set. He also capitalized on Paul’s double-fault in tiebreaker. Paul converted only one of his four break points.

Fritz will next play Australian John Millman. He defeated fourth-seeded Aslan Kartsev, 6-3, and 6-2. The Kremlin Cup was won by Karatsev on Sunday, but the Russian lost his cool at set 2-2. After sailing wide at deuce, he gave Millman a break. He was penalized one point for hitting his racket on court.

Karatsev was also disappointed by the loss.

This year, Millman is currently 0-5 in quarterfinal matches.

Botic van de Zandschulp, a Dutch qualifier, defeated eighth-seeded Sebastian Korda from the United States to set up a quarterfinal match against Andrey Rublev, their home favourite.

Rublev lost to Van de Zandschulp in Rotterdam's qualifying rounds on February 2017. Rublev, now sixth in the rankings, was still not in the top 100.

"These days, it's a little different. In his post-match press conference, Van de Zandschulp, ranked 69th, stated that the tables were "a little bit turned". "I am eager to play tomorrow and we will see," he said.

Van de Zandschulp was a quarterfinalist in the U.S. Open and broke the 38th-ranked Korda instantly. He also said that he didn't give him any free points, which limited the American to just two chances of breaking.

Croatia's Marin Cilic will face Spain's Roberto Bautista Agut in Friday's quarterfinals. Cilic defeated sixth-seeded Karen Khachanov from Russia 6-4, 6/7 (1), 6-4. Bautista Agut, third, beat Mackenzie McDonald 6-3, 2 2.