However, illness rather than injury appeared to be the problem.
Sinner was only on court for 24 minutes, but was out of sorts in hot conditions in Ohio and broken three times by rival Alcaraz to trail 5-0.
With Alcaraz about to serve for the set, four-time grand slam champion Sinner called for the trainer and decided to quit to hand the world No.2 a maiden Cincinnati Open title.
Sinner addressed the crowd after his retirement, saying: "I'm super, super sorry to disappoint you.
"From yesterday I didn't feel great, I thought I would improve during the night, but 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."
The Spaniard wrote "sorry Jannik" on the camera before he briefly consoled the Italian.
"Well, yeah it is not the way I want to win matches, to win trophies and I have to say sorry. I can't understand how he feels right now," Alcaraz said.
"I said many, many times you are a true champion and I am pretty sure from this situation you will come back better and stronger like you always do."
Sinner, who turned 24 on Saturday, was on 12-match winning streak and had won 26 straight matches on hard courts.
Alcaraz now holds a 9-5 advantage in his matches between the pair.
with AP
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