Top seed Dominic Thiem is through to his 12th ATP Tour quarter-final of the year after beating Jan-Lennard Struff to reach the last eight at the St Petersburg Open.

The Austrian recorded his 45th win on the circuit for the third straight year and equalled Rafael Nadal and Alexander Zverev for the most in 2018 with a 7-6 (7-4) 6-4 success over Struff.

Struff had three break points in the second set but was unable to convert any of them as Thiem set up a quarter-final clash with Daniil Medvedev, who beat Mikhail Kukushkin.

Mikhail Youzhny's final ever tournament ended at the last-16 stage as Roberto Bautista Agut knocked out the Russian in his 499th tour-level match.

Youzhny, 36, had already confirmed the St Petersburg Open would be his final competition before bowing out and Agut ended his career with a 7-6 (8-6) 3-6 6-3 victory.

"From the beginning until the end, I was professional," Youzhny said in quotes published on the ATP's website.

"The level of my game in the last match shows that I'm retiring actually at a very good level."

Defending champion Damir Dzumhur also advanced to the quarter-finals, where he will face Stan Wawrinka.

Second seed Stefanos Tsitsipas was eliminated at the Moselle Open as Lithuanian Ricardas Berankis secured a come-from-behind 6-7 (5-7) 7-6 (8-6) 6-3 win.

Seventh seed Filip Krajinovic and Benoit Paire were also ousted but fifth seed Nikoloz Basilashvili came from a set down to win.