Carlos Alcaraz has won the Cincinnati Open for the first time in his career after the world No 1, Jannik Sinner, his great rival, was forced to retire from their highly anticipated final with illness while trailing 0-5 in the opening set.

Sinner, who had looked under the weather from the start of the match, explained later that he had been struggling with illness since Sunday. “I’m super sorry to disappoint you,” the Italian said.

“From yesterday I didn’t feel great. I thought that I would improve during the night. It came up worse. I tried to come out and make it at least a small match but I couldn’t handle more. I’m very, very sorry for all of you. I know some of you on Monday maybe had to work or do something else, so I’m very, very sorry.”

In 31C (87F) heat and 57% humidity, extremely difficult conditions but temperatures they have both had to navigate since the early rounds, Sinner looked completely off from the beginning and his condition only appeared to worsen during his five games on the court.

Sinner opened the match with a sloppy opening service game, missing three of his first four first serves as Alcaraz eased to an early break. From the start of the set, Sinner looked determined to keep points as short as possible as he lashed out at forehands and accordingly committed increasingly wild unforced errors. Between points, Sinner walked gingerly and his face looked flushed. As the set wore on, his movement and leg drive on his strokes deteriorated.

Carlos Alcaraz celebrates his victory. Photograph: Matthew Stockman/Getty Images

Alcaraz has not always handled himself well when facing ailing opponents, including on Saturday night when he struggled briefly against a sick Alexander Zverev. To his credit, Alcaraz offered Sinner no free points, maintained his own intensity and made himself look incredibly difficult to beat. Down 0-4, Sinner requested the trainer out on the court but after serving a double fault to fall 0-5 behind, he decided it was time to end the match.

After the pair embraced at the net, Alcaraz walked over to Sinner’s bench and put an arm around his rival’s shoulders. Sinner then explained to Alcaraz that he had not been feeling well for some time but he had “tried to come out for the fans”. Alcaraz responded: “Don’t worry at all.”

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Two years ago, Alcaraz lost an incredible match against Novak Djokovic in the Cincinnati final, a match that is widely considered one of the greatest three-set matches of all time. Although he returned to take the title, this was not how he imagined it. With the trophy in his hands, Alcaraz appeared lost for words as he addressed the crowd. “As you said, Jannik, this is not the way I want to win matches, to win the trophy. I just gotta say sorry. I know and understand how you can feel right now. I can’t say anything you don’t know already, but as I say many times you are truly a champion. I’m pretty sure from this situation you’re going to come back stronger. You always do – that’s what true champions do. You really are one. Sorry and come back stronger.”

Alcaraz has now won eight ATP Masters 1000 titles by the age of 22 – only Rafael Nadal achieved eight Masters 1000 titles at a younger age. Sinner’s retirement ends his 26‑match winning run on hard courts.