Nicolas Almagro rallied from a set behind to beat defending champion Juan Monaco 4-6 6-0 6-3 and reach the last four of the German Tennis Championships in Hamburg.
Victory for the third-seeded Spaniard proved sweet revenge after the 29-year-old beat him in the last four of the tournament last year.
Meanwhile, qualifier Federico Delbonis from Argentina caused a major surprise by defeating 14th-seeded Fernando Verdasco 6-7 (5) 7-6 (8) 6-4 to make the semis in northern Germany for the second time in his career.
Delbonis, whose previous semi-final appearance was two years ago on clay in Stuttgart, grabbed the reward of a semi-final tie with Roger Federer.
After fending off a pair of match points in the second set, Delbonis failed to serve out the match at 5-3. He then wasted two match points on Verdasco's serve before the Spaniard double-faulted on the third.
Federer made his way into the final four by defeating Germany's Florian Mayer 7-6 3-6 7-5 in a hard-fought contest.
With the only break point of the first set being wasted by Mayer, the German felt he had missed a great opportunity.
"I have only myself to blame," said Mayer. "Roger wasn't in his best form. I let a great chance off the hook and was almost afraid of winning against a big name."
In the days other quarter-final match, home favourite Tommy Haas was beaten in straight sets by Fabio Fognini from Italy.