Eighth-seeded David Goffin of Belgium rallied to beat top-seeded Jo-Wilfried Tsonga 1-6, 7-6 (5), 7-5 on Friday to reach the Moselle Open semi-finals.
Goffin next plays Germany's Jan-Lennard Struff for the first time. Struff advanced after third-seeded Philipp Kohlschreiber retired when trailing 5-0 after just 20 minutes.
Second-seeded Gael Monfils of France also went through to the last four, hitting 12 aces in a convincing 6-3, 6-4 win against seventh-seeded Jerzy Janowicz of Poland. Monfils did not face a single break point, while breaking the Polish player's serve twice.
He next faces No. 6 Joao Sousa of Portugal, who got the better of Frenchman Paul-Henri Mathieu 3-6, 6-3, 7-6 (4) in their first meeting.
Looking to win the Moselle title for the third time, Tsonga dominated the first set, hitting five aces and converting both of his break points.
The players traded breaks in the third set before Tsonga saved two break points at 15-40 down in the 11th game. But he missed a chance to win it and Goffin seized his next opportunity before serving out victory on his second match point.
The 45th-ranked Goffin's only career title was on outdoor clay in Kitzbuehel, Austria in August, while the 61st-ranked Struff plays his third semifinal of the season, having lost to Italy's Fabio Fognini on outdoor clay in Munich and to Tsonga on hard courts in Marseille.
Kohlschreiber, meanwhile, was clearly in difficulty and had already dropped serve three times when he stopped.