Jannik Sinner was bidding to become the first man since Roger Federer in 2015 to win back-to-back Cincinnati titles, but he looked uncomfortable early on and called it quits after 23 minutes of play.
Carlos Alcaraz won the Cincinnati Open after top seed Jannik Sinner retired as he was feeling unwell on Monday, raising concerns about the Italian's fitness just days before he is due to begin the defence of his US Open title.
The Spanish second seed snapped the world number one's 26-match-winning streak on hardcourts when his ailing rival signalled he could no longer continue while trailing 5-0 in the first set of the final.
"This is not the way that I want to win trophies, I just have to say sorry, I can understand how you must feel now," Alcaraz told Sinner during the presentation ceremony after capturing his third Masters 1000 crown of the season following his success in Monte Carlo and Rome.
"As I said many times, you are a true champion and I am sure from these situations, you are going to come back even stronger, as you always do. That’s what true champions do."
Sinner had been bidding to become the first man since Roger Federer in 2015 to win back-to-back Cincinnati titles but he looked uncomfortable early on in the sweltering conditions.
He called it quits after 23 minutes of play.
"I'm super, super sorry to disappoint you," Sinner told the crowd.
"Yesterday I didn't feel great. I thought that I would improve during the night, but it came up worse.
"I tried to come out, tried to make it at least a small match, but I couldn’t handle more, so I’m very sorry."
It was also unclear whether the Italian would be fit enough to partner Katerina Siniakova in the revamped mixed doubles event at Flushing Meadows which is due to take place on Tuesday and Wednesday.
Singles action at the hardcourt major begins on Sunday.
Monday's clash had been expected to be a dress-rehearsal ahead of a potential final showdown between the world's two best players in New York, with the duo having faced off in the title match at the last two majors.
Alcaraz came from two sets down and saved three championship points to triumph at Roland Garros, while Sinner emerged victorious at Wimbledon.
Monday's result extended Alcaraz's head-to-head record over Sinner to 9–5, including a 6–2 edge on hardcourts.
