Struff was in an enticing match against American tennis player Marcos Giron that ended 7-5, 5-7, 7-6 (10-8).
His first title success came in July 2018, when he made it into the main draw of the German Open and won the tournament.
He has won three ATP doubles titles and reached one ATP singles final. Struff reached the semifinals of the ATP Tour for the first time in Marseille
Felix Auger-Aliassime upset World No.3 Alexander Zverev to carry Canada through to the ATP Cup semi-finals on Thursday.
The sixth seed needed two tie-breakers to get the better of the Russian qualifier and enter the third round in Paris.
Meanwhile, Hugo Nys and Tim Puetz bag their maiden ATP Tour team title at the Millennium Estoril Open.
Elsewhere, Cameron Norrie downs Marin Cilic in a tight contest to enter Estoril final.
The Belarusian came from a set down to defeat the German favourite in a three-set thriller to set up a semifinal clash against Jan-Lennard Struff.
Sander Gille and Joran Vliegen overcome a marathon battle to reach the BMW Open final in Munich.