Trouble for Ruud as O'Connell proves the Cincinnati Kid

Chris O'Connell was in no mood to celebrate at the Cincinnati Open after moving through to the round-of-32 following a concerning injury to his 11th seeded opponent Casper Ruud with the US Open approaching.

The Australian qualifier picked up his fourth win in five days at the Masters 1000 tournament on Sunday in what's proving his most profitable week of the season -- but there were no smiles as he sympathised with Norway's three-time grand slam finalist on his retirement after 64 minutes play.

Ruud had started well, moving into a 5-2 lead in the opening set, but the Sydneysider powered back to take the first set, sealing it 7-5 with an ace, as Ruud began to look in increasing discomfort with a back problem.

With the second set going with serve, Ruud leading 2-1, the Norwegian world No.17 stretched for a wide serve and immediately decided enough was enough, going to the net to offer his hand to the Aussie.

Clearly with one eye on the US Open, where the main draw starts a week on Sunday, Ruud, a finalist at Flushing Meadows back in 2022, wasn't evidently not prepared to take any risks.

For world No.129 O'Connell, though, it was the continuation of an excellent spell following his first-round victory over Poland's world No.68 Kamil Majchrzak.

He'll next meet Brazilian teenage superstar Joao Fonseca, who defeated Botic van de Zandschulp 6-4 7-6 (7-2), and victory in that match would push O'Connell back to the verge of the world's top 100.

He's the third Australian to make the third round along with Rinky Hijikata, who faces Jakub Mensik, and national No.1 Alex de Minaur, who meets British Wimbledon hero Arthur Fery.