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The No.2 seed, who triumphed in Rome in 2017 and 2024, won 7-5 6-3 in one hour and 44 minutes, with Altmaier providing him with a stern test.

After a rain delay of more than 90 minutes pushed back the match's start time, Zverev went a break up in the sixth game of the opener, though Altmaier did what so many players fail to do and caused the world No.3 difficulties on his serve.

Altmaier secured a commanding break in game nine and looked to be on course for a tie-break, only for Zverev to break again at the last opportunity.

Zverev could be heard frantically asking his team if further rain was forecast after taking the opening set, as a drawn-out encounter appeared to be on the cards.

The second set did last 50 minutes, with Altmaier striking the first blow with a break in game three. Zverev hit back immediately, and the frustration got the better of his opponent as he was warned for comically gripping the head of his racquet and striking the ball directly into the air using the handle.

After saving three break points in game seven, Zverev broke again in game eight, and he needed three match points to get it done on serve, finishing with a wonderful downward backhand after an exchange at the net.

Earlier, Casper Ruud defeated Zachary Svajda 6-1 6-3 and Alex De Minaur suffered another disappointing exit against a home favourite, Matteo Arnaldi.

Rafael Jodar, who beat De Minaur in Madrid, continued his momentum from his quarter-final run in Spain, beating Nuno Borges 7-6 (7-4) 6-4 to increase his ATP Tour-level record on clay to 13-2.