Diego Schwartzman made an unforced error after his big fourth-round victory Monday over No. 6 seed Alexander Zverev.

He thought his next match in the quarterfinals was against the Italian, Matteo Berrettini. During the on-court interview, the 5-foot-7 Schwartzman didn’t realize he was lined up to play Rafael Nadal if he advanced — only the second-best player in tennis history.

Oops.

“I was thinking OK the next one is Berrettini,’’ Schwartzman said. “I lost against [Berrettini] in Wimbledon with three match points. So, OK, I have him again.”

Except he doesn’t. Schwartzman gets Nadal in Wednesday night’s quarterfinals. Ironically, the 27-year-old Argentinian trains at Nadal’s academy in Mallorca, Spain. However, he has an 0-7 record against Nadal. Schwartzman relies on being one of the game’s best service returners.

“I need to steady my games against him,’’ Schwartzman said.

The 20th seed will be the heavy underdog in his second Open quarterfinal after making it in 2017. He lost to Nadal in the second round in 2015.

Schwartzman has an interesting background. According to tennis sources, Jewish relatives escaped Poland during the Holocaust and wound up in Argentina, where an enclave of Holocaust survivors live.

Through Labor Day, the Open set an attendance record. After eight days, the Open had drawn 540,333 fans. The previous record was 538,266, set in 2018. One reason for the two records is the Open started selling separate night-card sessions for Louis Armstrong Stadium.

The doubles team of Victoria Azarenka-Ashleigh Barty moved into the women’s semifinal Tuesday. They had knocked off so-called McCoco (Coco Gauff-Caty McNally) in the prior round. Azarenka-Barty rallied to beat Timeo Babos-Kristina Mladenovic 2-6 7-5, 6-1.

In the Junior Open, Northport’s Cannon Kingsley upset fifth seed Thiago Austin Tiranti 7-6, 2-0 (retired) in the second round. … The German duo of Kevin Krawietz-Andreas Mies, which won the New York Open and the French Open, moved to the men’s doubles semifinals with a 7-6, 6-4 victory over Leonardo Mayer-Joao Sousa.