An emotional Jannik Sinner retired from the Cincinnati Open final against Carlos Alcaraz with illness, only six days before he begins his US Open title defence.

World number one Sinner was out of sorts from the first point of the highly anticipated showpiece, and trailed 5-0 after 23 minutes on another hot day.

He sat with ice on his head at the first changeover after going a double break down, and called for the trainer after losing another two games.

After a brief chat, Sinner apologised to the umpire and officials, growing tearful as he told them he "felt really bad" for the fans.

The Italian shared a hug with Alcaraz and walked out to the middle of the court, raising his hands in apology to the crowd before returning to his seat and hiding his head in his towel.

Spanish world number two Alcaraz quickly went over to comfort his rival, putting an arm around him and speaking to him before writing on the camera lens: "Sorry Jannik".

Sinner told the crowd, "I'm super sorry to disappoint you. From yesterday, I didn't feel great, I thought I would improve but it came up worse.

"I tried to come out, to make it at least a small match, but I couldn't handle more.

"I am very sorry for all of you. I know that maybe some of you, on a Monday, you had to work or do something else, so I'm really sorry."

It is a first Cincinnati title for Alcaraz and a seventh triumph of the season overall - but as he said afterwards, this is not the way he would have wanted to win.

"I can't say anything you don't know already. You will come back even stronger like you always do," Alcaraz said to Sinner.

"That's what true champions do and you are one."

Sinner, Alcaraz and women's finalist Iga Swiatek are all due to play in the US Open mixed doubles on Tuesday in New York - but it seems unlikely Sinner will now pair up with Katerina Siniakova as planned.

The singles main draw at Flushing Meadows begins on Sunday.

Despite his obvious disappointment at not being able to complete the match, Sinner can take some comfort that it was illness, not injury, that held him back.

He was on a 26-match winning streak on hard courts going into the final and had not dropped a set in Cincinnati.

Sinner will stay world number one regardless of the loss, but Alcaraz is closing in on the top ranking - and remains Sinner's biggest rival for the US Open title.

The pair can only meet in the final in New York, and Sinner is defending more ranking points than Alcaraz, who lost in the second round last year.

This was the fourth time the pair have met in a final this year, with Alcaraz winning twice on clay and Sinner triumphing in their last meeting at Wimbledon.

However, this was barely a contest, with Sinner's lack of movement and early attempts to hit his way out of trouble hinting that something was amiss.

Alcaraz won 21 of the 29 points on offer and committed just one unforced error to Sinner's nine.

The pair have split the past seven Grand Slams between them, and it is difficult to see who will stop them from adding an eighth to that tally.