Casper Ruud took his time finding his rhythm as he beat Alex Molcan 3-6, 6-4, 6-4 to reach the quarterfinals at the Munich Open on Thursday.

Molcan built a 4-0 lead in the first set as Ruud struggled with his serve, but the Norwegian clay-court specialist fought his way back and broke Molcan at 4-4 in the decider before serving out the match.

“It was not the best start for me but I was able to find my rhythm a little more as the match went on," Ruud said. “(Molcan is) a tough player and good confidence for me to get these types of wins.”

The 23-year-old Ruud said he was especially pleased to be playing well at the comparatively high altitude of Munich as preparation for similar conditions at the Madrid Open next week.

READ: Halep and Badosa to meet in second round of Madrid Open

It was Ruud's opening match in Munich after getting a first-round bye. At a career-high No. 7, the Norwegian is the highest-ranked player left in the draw after Alexander Zverev's surprise loss to 18-year-old Holger Rune on Wednesday.

Ruud will face eighth-seeded Botic van de Zandschulp in the quarterfinals after the Dutch player defeated Egor Gerasimov 6-2, 6-3.

Germany's Oscar Otte managed the surprise of the day by beating Reilly Opelka of the United States 7-6 (4), 6-2. Opelka had two set points in the first set but Otte saved them both before taking the set to a tiebreaker.

Otte's quarterfinal opponent is Alejandro Tabilo, who was leading 6-2, 2-6, 4-1 when Hugo Gaston retired from their second-round match. Otte cruised past Tabilo 6-0, 6-3 when they played last week at the Serbia Open.

Auger-Aliassime, Korda set up quarterfinal clash in Estoril

Top-seeded Canadian Felix Auger-Aliassime defeated Spain's Carlos Taberner 1-6, 6-2, 6-2 in the second round of the Estoril Open.

In the last eight match, Auger-Aliassime will face eighth-seeded American youngster Sebastian Korda who won 6-3, 6-3 against Frenchman Benjamin Bonzi in the second round.

Other quarterfinals - an all-Spanish final between Albert Ramos-Vinolas and Fernando Verdasco, Monte Carlo Masters runner-up Alejandro Davidocih Fokina will be up against Korda's countryman Francis Tiafoe and France's Richard Gasquet will play Argentine Sebastian Baez.