Roddick won eight of the final nine games en route to a 7-5, 6-1 victory over former Wimbledon champion Richard Krajicek of the Netherlands in this claycourt event.
"In the second set I played solid, clean tennis," Roddick said. "I had my chances and I took advantage of them, that's the difference between average tennis and good tennis. I'm happy to be in the semifinals here. Until now, I hadn't won two matches in a row on the clay here in Europe, so this has been a good week."
The 20-year-old Roddick lost in the final of the U.S. Men's Clay Court Championship last month, but was eliminated in the second round at the Rome Masters and Hamburg Masters. The sixth seed at next week's French Open, Roddick takes on eighth-seeded David Sanchez of Spain in the semifinals.
Sanchez, who won a title on clay at Vina del Mar, Chile in February, coasted to a 6-1, 6-4 win over 19-year-old Croatian Mario Ancic.
"I think I played pretty solid," Sanchez said. "Today the key for me was my serve. I think I had a very high percentage of first serves and when you get the first in you can take the initiative on points."
Russian sixth seed Nikolay Davydenko overcame some early mistakes and posted a 7-5, 6-1 victory over Anthony Dupuis. Davydenko, seeking his third title of the year, will take on Dutchman Martin Verkerk.
"I was making a lot of mistakes when I got up 3-1 and when I found myself down, 5-3, I was trying to put more balls in the court," Davydenko said. "I kept fighting, cut down on my mistakes and started playing better and better. I just rolled from there."
Verkerk had the toughest match of the day as he got past Brazil's Flavio Saretta, 7-6 (7-4), 7-6 (7-2).
"I think the key was those two tiebreaks and playing good tennis at those moments," Verkerk said. "I was really happy with the way I played. Obviously I play better on fast surfaces and my game suits that, but I always practiced on clay courts when I was growing up, so I am comfortable on it as well."