“We cannot forget also about Blockx. He’s a bit under the radar, but is he an incredible player,” Jannik Sinner told reporters at the Mutua Madrid Open on Wednesday – a timely reminder that came just a day before the 21-year-old Alexander Blockx pulled off yet another upset to reach the semi-finals at the Caja Magica.

After knocking out 16th-seeded Francisco Cerundolo, third-seeded Felix Auger-Aliassime, and 28th-seeded Brandon Nakashima in the previous three rounds, Blockx kept up his giant-slaying ways by ending the title defence of former world No.2 Casper Ruud 6-4, 6-4 on Thursday to reach the first tour-level semi-final of his young career.

The 2023 Australian Open junior champion had never won a tour-level match on clay prior to this month but has since amassed a 10-2 record on the red dirt through qualifying and main draws at ATP level.

Blockx arrived in Madrid at a career-high ranking of 69 and is now up to 35 in the live ATP rankings.

“To be honest, I don’t know, I’m just happy being here,” an elated Blockx told Tennis TVfollowing his 95-minute victory over the 12th-seeded Ruud.

“Even winning my first match here [against lucky loser Cristian Garin], I barely escaped in the first round. 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.”

Showcasing his explosive brand of tennis, Blockx took the initiative and broke Ruud to build a 3-1 lead before the Norwegian steadied the ship and broke back for 4-4.

For a 1.93cm player, Blockx is a great mover and he utilised that well to strike again as he secured the 51-minute opening set on his fourth opportunity.

A crucial break in game seven in the second set gave the Belgian the edge he needed to take control of the match and he was soon celebrating on Manolo Santana Stadium, where he will face two-time champion Alexander Zverev or 10th-seeded Flavio Cobollo in the semi-finals on Friday.

“I think the conditions suit me well here. I feel like it’s clay, which is slow, which gives me time to settle and go for my shots, but at the same time it’s quite fast with the altitude and with the heat sometimes. So I think it’s the perfect combination for me,” said Blockx.

“Always before the match, I’m very curious about what’s going to happen, like am I going to feel my ball well? How is the opponent going to play? But I felt from the first game that I was hitting my ball quite well, I made every return. Once I got my first game, I was really into the match.”

Blockx is now 4-2 against top-20 opposition, with all four victories coming this month.

Born in Antwerp to Ukrainian parents who were both professional athletes (father Oleg ran track and mother Natalia was a swimmer), Blockx picked up a tennis racquet when he was just four years old.

Ruud paid tribute to Blockx after their quarter-final clash, saying: “I found it depressing this game, because he didn’t have really any weaknesses.”

The Norwegian, who will vacate the top 20 in the rankings for the first time since May 2021, said he rewatched several of Blockx’s matches and highlights to scout his opponent ahead of the match.

“Ultimately, when you’re out there, you can scout all you want, but you have to hit the shots that you want to hit. Today I think I was a little bit too loose with my forehand. I went for some bigger shots and winners that sailed a bit long or sailed a bit wide. I think ultimately that gave him, maybe not the win, but I didn’t get the best start. He got an early break, building confidence probably in his game,” Ruud added.

“I think I was impressed because, like I said also before, didn’t really find any big weakness or hole in his game. And when you don’t do that, it’s easier to kind of over-hit and feel like you don’t get anything out of your own shots. So credits to him for being, obviously, very good serve, yes, but also very solid and playing well from the baseline.”

The Mutua Madrid Open has featured several breakout performances from members of the younger generation like Blockx and Rafael Jodar, with many predicting they, along with Joao Fonseca, will be leading the charge against dominant duo Jannik Sinner and Carlos Alcaraz.

“We will definitely try,” said Blockx.

“I think they are still a big level ahead of us. But, yeah, that’s why we play this sport, to be able to make a chance maybe to compete with them. It will be really difficult, but if you believe in it then you have a small chance.”