Cincinnati, Aug 11 : Ben Shelton eased into the third round of the Cincinnati Open after Camilo Ugo Carabelli retired early in the second set with a knee injury.

Competing just three days after winning his first ATP Masters 1000 title in Toronto, Shelton was leading 6-3, 3-1 after 64 minutes when the Argentine retired.

Carabelli grasped his knee on the first point of the fourth game of the second set after an awkward movement coming forward into the court. After a medical timeout he could not move freely and retired after completing Shelton's service game, ATP reports.

"I feel for Camilo and it's obviously not the way that you want to go through. I hope that he has a quick recovery," Shelton next faces Spanish veteran Roberto Bautista Agut, said.

Meanwhile, world No. 3 Alexander Zverev dropped a serving masterclass on American Nishesh Basavareddy to breeze into the third round with a 6-3, 6-3 win, his 40th of the year.

Zverev said that he was happy to get through without expending too much energy, and to avoid playing in the heat of the day.

"I requested (a night match), coming from Toronto, where it's a lot colder, I need some days getting used to the heat and conditions. I'm happy with the win and happy with how I played."

The two-time ATP Finals champion will next play 27th seed, American Brandon Nakashima, who toughed out a 4-6, 6-3, 7-6(5) win over Belgian qualifier Alexander Blockx.

Elsewhere, Karen Khachanov, who fell to Shelton in a third-set tie-break in the Toronto final, won his opening Cincinnati match against French qualifier Valentin Royer 6-4, 7-6.

The World No. 12 hit seven aces according to ATP Stats and took charge with his forehand in his one hour, 50-minute win. He next faces American wild card Jenson Brooksby.

/IANS