(Reuters) -Carlos Alcaraz won the Cincinnati Open after top seed Jannik Sinner retired while trailing 5-0 in the first set of their final on Monday.

The Spanish second seed collected his third Masters 1000 title of the season, having won in Monte Carlo and Rome.

Sinner, who looked uncomfortable early on in the sweltering conditions, pulled out after 23 minutes of play, raising concern about his fitness just days before he is due to begin the defence of his U.S. Open title.

Singles action at the hardcourt major begins on Sunday.

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.

(Reporting by Janina Nuno Rios in Mexico City, editing by Prit

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