After reclaiming No. 1, Carlos Alcaraz tumbles out of Rome to 135th-ranked Fabian Marozsan

WATCH: Marozan won the final six points of the match to clinch the upset of the tournament.

Carlos Alcaraz endured one of the most shocking losses of his young career on Monday, tumbling out of the Internazionali BNL d’Itali, 6-3, 7-6 (4), to Hungarian qualifier Fabian Marozsan, ranked No. 135.

Alcaraz had already assured himself a return to No. 1 on the ATP rankings, surpassing defending champion Novak Djokovic by competing and winning his opening round on Saturday, but saw a 4-1 lead in the second set tiebreaker evaporate as the 23-year-old Marozsan continued a fairytale week at the Foro Italico.

“It’s not easy to say something,” he said on court after the match. “I’m very happy. I can’t imagine this one. It was my dream last night, but it’s true! I’m very happy. Yesterday, we spoke about this match and I said I’d just try to do something special to try and win a few games, maybe a set. I just beat the world No. 1; he’s our best in the sport so I’m really happy about this.”

Playing his first Masters 1000 main draw, Marozsan booked the encounter with Alcaraz by knocking out an in-form Jiri Lehecka in a third-set tiebreaker, and was fast out of the gates against the No. 2 seed on Monday with a 5-2 lead.

Alcaraz was coming off back-to-back victories in Barcelona and Madrid, found himself playing catch-up in the second set exchanging breaks with the Hungarian and saving a break point that would have seen Marozsan serve for the match.

Forcing a tiebreaker, Alcaraz looked to have finally found a groove with the early mini-break, but Marozsan rallied to win the final six points of the match—pressing the reigning US Open champion into back-to-back forehand errors to book his spot in the fourth round.

“It’s amazing,” Marozsan said. “I don’t know what happened during the point; I just tried to hit back every ball and tried to do my best. I tried to find something, how I can win points against him in this tough situation, and it’s just happened. I don’t know what to say.

“Everything was perfect today: the crowd, the weather, the courts. I’m just very happy to be doing my job.”

Waiting for him in what is undoubtedly a much more open quarter is No. 15 seed and reigning Western & Southern Open champion Borna Coric, who also edged into the round of 16 by beating a Spaniard, Roberto Carballés Baena, in straight sets.

It will be a match to forget for Alcaraz, who struck 24 errors to just 12 winners. For Marozsan, who ended with mirror opposite stats (24 winners to 13 errors) it will be a match he won't soon forget.