After knocking out top seed Daniil Medvedev in the previous round at the Moselle Open, Stan Wawrinka's stunning run of form continued with an epic 6-4, 4-6, 7-6 (5) victory over Mikael Ymer on Friday to reach the last four.
The pair battled for two hours and 47 minutes as 37-year-old Wawrinka rolled back the years to take the victory.
Once ranked as high as No. 3 in the world, Wawrinka has been plagued by injuries and has slumped to world No. 284.
Elsewhere, No. 2 seed Hubert Hurkacz moved a step closer to the successful defence his Moselle Open title with a 6-3, 6-2 victory over Frenchman Arthur Rinderknech.
Hurkacz, who is aiming for his sixth career ATP title, will next face Italy's Lorenzo Sonego, seeded 18th, in the semifinals.
Sonego advanced to the semifinals following a routine 6-4, 6-4 win over American Sebastian Korda.
Later, Alexander Bublik edged Holger Rune in two hours, one minute to set up a semifinal tie with Wawrinka.