Blockx, 21, who broke into the top 100 for the first time last month and is at a career-high 69 in the world, has knocked out four consecutive seeds, including third-seeded Felix Auger-Aliassime, en route to the final four.
Blockx is through to his first tour-level semi-final and is the first Belgian man in tournament history to make it this far at the Madrid Open.
Prior to this month, he had never played a tour-level match on clay. He now enters the semi-finals with a 10-2 mark on the red dirt through qualifying and main draw matches at ATP level.
"To be honest, I don't know, I'm just happy being here. Even winning my first match here, I barely escaped in the first round," said Blockx in his on-court interview.
"I was happy about that already, but semi-finals is something I wouldn't have even dreamed of to begin with. I'm proud with how I'm playing the last couple of matches."
Blockx drew first blood, breaking in game three en route to a 3-1 lead.
Norwegian Ruud began dictating with his forehand and closed the gap to level for 4-4, but Blockx was unfazed and struck again to regain his advantage, and he secured the set on his fourth opportunity after 51 minutes of play.
Blockx made his move in the seventh game of set number two to carve a 5-3 opening, and after squandering two match points on Ruud’s serve, he closed it out on his own serve to secure the win in 96 minutes.
Ruud committed 17 unforced forehand errors during the match and will drop out of the top 20 for the first time since May 2021 when the new rankings are released next week.
"I found it depressing this game, because he didn't have really any weaknesses," Ruud said after the match.
The three-time Grand Slam finalist revealed he had been dealing with a calf issue since Monte Carlo and is happy with how his leg held up this fortnight in the Spanish capital.
Zverev cruises past Cobolli
Two-time Madrid champion Zverev reached a seventh semi-final from his last eight Masters 1000 tournaments with a 6-1, 6-4 win against Italian 10th seed Flavio Cobolli, who beat the German en route to the Munich final 12 days ago.
"No revenge, I love Fabio, he's one of my favourite guys on the tour... This is sport, sport can change very, very quickly. I played an amazing first set, he didn't," Zverev said.