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The defending champion made a serene start as he hunted a fifth appearance in the last four at Munich, but Cerundolo made things interesting with a stunning turnaround in the opening set.

Having gone 4-1 down, the Argentinean immediately reeled off three straight games, including two on Zverev's serve, to draw level before earning a third and decisive break in game 12.

At that moment, it looked like Cerundolo, who had never lost to Zverev on clay in three such meetings, might be set to advance, but the top seed had other ideas.

Zverev upped the intensity, with a series of hard-fought breaks allowing him to record a bagel in the second set and force a decider.

Two more breaks – in games three and seven – proved enough for Zverev to take the third, and he did not allow his opponent a sniff against his serve, not facing a break point.

Speaking after his triumph, the German said: "I thought it was an incredible level. Even in the first set when I was up 4-1, he started playing unbelievably; he was returning unbelievably.

"My first-serve percentage was very high, and he started returning a foot in front of the baseline. So there was nothing I could do, to be honest.

"I was happy with my level even in the first set. Definitely happy to get the win today against Francisco, for the first time on clay."

Zverev will now face Flavio Cobolli for a spot in the final after the Italian beat Czechia's Vit Kopriva 6-3 6-2.

At the Barcelona Open, Andrey Rublev and Hamad Medjedovic both advanced to the semi-finals with straight-sets wins, over Tomas Machac and Nuno Borges, respectively.