ST. PETERSBURG, Russia — Top-seeded Dominic Thiem reached the quarterfinals of the St. Petersburg Open with a 7-6 (4), 6-4 win over Jan-Lennard Struff on Thursday.

The Austrian player's next opponent will be eighth-seeded Daniil Medvedev of Russia, who prevailed against Mikhail Kukushkin 6-3, 6-4.

Russian veteran Mikhail Youzhny retired from tennis after losing 7-6 (6), 3-6, 6-3 to fifth-seeded Roberto Bautista Agut of Spain.

The 36-year-old Youzhny won 10 tour titles and reached a career-high ranking of eighth in 2008.

Defending champion Damir Dzumhur of Bosnia progressed by beating Guido Pella 6-4, 6-4.

Dzumhur landed 70 percent of his first serves as he set up a meeting with Stan Wawrinka.

Nineteen-year-old Canadian Denis Shapovalov defeated Matteo Berrettini of Italy 7-6 (6), 4-6, 6-0. He next plays Martin Klizan.