Fancied Carlos Alcaraz stunned by Hungarian qualifier Fabian Marozsan in Rome
He had come into the tournament on the back of triumphs in Barcelona and Madrid and will replace Novak Djokovic (35) as the world number one for top billing at Roland Garros which begins later this month.
But Alcaraz was far from his best and got off to a shaky start in his second match in Rome, dropping serve to love in the fourth game as world number 135 Marozsan built a big lead and dominated the late exchanges to easily win the opening set.
Marozsan continued to heap the pressure and broke for a 4-3 lead after Alcaraz saved three breakpoints earlier in the second set, but the Budapest native immediately let his opponent off the hook before forcing a tiebreak.
Alcaraz raced to a 4-1 lead but Marozsan hit back in superb style to reel off six points and hand the world number two only his second defeat on clay in 22 matches this season.
"It's not easy to say something," Marozsan said immediately after the biggest win of his career. "I'm very happy, I could have never imagined this. It was my dream last night.
"I said I'd try to do something special by winning a few games and maybe a set and I just beat the (soon to be) world number one, the best player in our sport now.
"Everything was perfect today."
It was the first defeat for Alcaraz to a player ranked outside the world's top 100 since the 2021 Paris Masters when he fell to Hugo Gaston (22).
World number three Daniil Medvedev (27) earned a hard-fought 3-6 6-1 6-3 win over Spain's Bernabe Zapata Miralles (26) to reach the last 16 in Rome for the first time.
"I'm definitely playing my best tennis on clay right now," said Medvedev, who reached the quarter-finals in Monte Carlo and the last 16 in Madrid.
"I have never played so well in Rome or in Madrid. I'm feeling great."
Sixth seed Andrey Rublev (25) also advanced after defeating Spaniard Alejandro Davidovich Fokina (23) 7-6(8) 6-3.
Qinwen Zheng (20) earlier beat Chinese compatriot Wang Xiyu (22) 6-4 3-6 6-1 to reach the quarter-finals in the women's draw while American Madison Keys (28) lost 2-6 6-2 6-4 against Ukraine's Anhelina Kalinina (26).
