David Goffin missed out on the chance of a third tour title this month after losing his European Open quarterfinal against Stefanos Tsitsipas.
Goffin was the top seed on home turf in Antwerp after back-to-back victories in Japan and Shenzhen but, despite a strong start, caved 2-6 7-6 (7/1) 7-6 (7/4) to his Greek opponent.
Fourth seed Diego Schwartzman awaits in the last four after seeing off David Ferrer 7-5 6-2.
That leaves second seed Jo-Wilfried Tsonga as the main attraction after he turned in a strong show to defeat fellow Frenchman Julien Benneateau 7-6 (7/5) 6-2.
Tsonga, with wins in Rotterdam, Marseille and Lyon in 2017, faces world number 98 Ruben Bemelmans.
Bemelmans' presence ensures one Belgian in the showpiece final after Goffin's stumble, with the 29-year-old beating Joao Sousa 4-6 7-6 (7/2) 6-4.
Adrian Mannarino remained on track for a first ATP tour title after sealing his place in the semi-finals of the Kremlin Cup.
The Frenchman is the highest remaining seed in Moscow, at third, and continued his progress with a 6-7 (6/8) 6-1 6-2 over Dudi Sela.
The world No. 29 has finished runner-up in Antalya and Japan this year.
Ricardas Berankis is all that stands between Mannarino and a final berth, after the Lithuanian defeated Alexander Bublik 7-5 6-1.
The other semifinal will be contested by Mirza Basic and Damir Dzumhur, both from Bosnia and Herzegovina.
Sixth seed Dzumhur will be favourite against the qualifier having come from behind to defeat Andreas Seppi 4-6 6-2 6-4.
Basic ousted the last of the draw's home contingent with a 3-6 6-4 6-1 win over Danil Medvedev.
At the Stockholm Open, top seed Grigor Dimitrov progressed to the semis with a regulation 6-3 6-4 victory over Mischa Zverev.
There will be no meeting with second seed Kevin Anderson, though, the latter having lost 7-6 (7/1) 7-6 (7/1) to Spaniard Fernando Verdasco.
Juan Martin Del Potro and Fabio Fognini make up a strong final four having dispensed wit Yuichi Sugita and Jack Sock, respectively.