Big-serving American Reilly Opelka called it “a fluke” after reaching his first Masters semifinal with a 7-5, 7-6 (2) win over Argentine qualifier Federico Delbonis at the Italian Open on Friday.

Not previously known for his clay-court tennis, the 6-foot-11 (2.11-meter) Opelka overwhelmed Delbonis with 18 aces to record his fourth straight-set win of the week.

“I’m surprised. Clay is not really my thing — not much of an American thing,” Opelka said. “It’s probably just a fluke. But I’m on with it.”

Reilly Opelka, of the United States, serves the basll during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Reilly Opelka, of the United States, serves the basll during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Opelka also saved all four break points he faced. But it wasn’t just his serve that made the difference.

The 47th-ranked Opelka hit a delicate slice volley drop-shot winner in the final game before the tiebreaker and then produced a difficult backhand cross-court passing shot to set up the only match point he needed.

Opelka entered Rome on a six-match losing streak that included dealing with a bout of COVID-19.

Argentina's Federico Delbonis returns the ball to Reilly Opelka, of the United States during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Argentina's Federico Delbonis returns the ball to Reilly Opelka, of the United States during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

“I wish I could use that as an excuse but the first 10 weeks of the year I just didn’t play well. It wasn’t COVID-related at all," Opelka said. "Obviously after you lose eight weeks in a row, the one thing you want to do is practice — not get COVID and be stuck inside for 2 ½ weeks. But I guess it worked out.”

Opelka’s semifinal opponent will be either nine-time Rome champion Rafael Nadal or 2017 winner Alexander Zverev.

In the women’s tournament, Petra Martic became the first semifinalist with a 7-5, 6-4 win over Jessica Pegula.

Croatia's Petra Martic, serves the ball to United States' Jessica Pegula during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Croatia's Petra Martic, serves the ball to United States' Jessica Pegula during their quarter-final match at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Martic, who recently hired 2010 French Open champion Francesca Schiavone as her coach, will face either 2019 Rome champion Karolina Pliskova or 2017 Roland Garros winner Jelena Ostapenko for a spot in the final.

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Croatia's Petra Martic celebrates after winning a point during her quarter-final match against United States' Jessica Pegula at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)

Croatia's Petra Martic celebrates after winning a point during her quarter-final match against United States' Jessica Pegula at the Italian Open tennis tournament, in Rome, Friday, May 14, 2021. (AP PhotoGregorio Borgia)