He will face sixth-seeded Casper Ruud of Norway in the quarterfinals Friday. They met at the Madrid Open in May, with Ruud winning in straight sets in the round of 16.
“I’m looking for a change, looking for a switch-up this time,” Tsitsipas said.
Ruud beat Dusan Lajovic of Serbia 6-4, 6-3.
Top-seeded Daniil Medvedev of Russia also advanced, beating Australia’s James Duckworth 6-2, 6-4 in a night match.
“I managed to get a few tight games, managed to get ahead,” Medvedev said. “I think he lost his rhythm a little bit, so that was well my fault, also,”
Medvedev will face seventh-seeded Hubert Hurkacz of Poland. Hurkacz edged Nikoloz Basilashvili of Georgia 6-7 (7), 6-4, 6-4.
Fourth-seeded John Isner of the United States beat Russia’s Andrey Rublev 7-5 7-6 (5). Isner will face 11th-seeded Gael Monfils of France, a 6-1, 7-6 (2) winner over American Frances Tiafoe
American Reilly Opelka beat Lloyd Harris of South Africa 4-6, 7-6 (6), 7-6 (4). Opelka will play 10th-seeded Roberto Bautista Agut of Spain. Bautista Agut overcame three match points to beat eighth-seeded Diego Schwartzman of Argentina 6-3, 3-6, 7-5.