Australian tennis firebrand Nick Kyrgios has slammed Kazakhstan's Alexander Bublik for an apparent slur that was picked up by microphones during his US Open first-round loss to Dominic Thiem.

Bublik went down 6-3, 6-2, 6-4 to the 2020 champion, and during the second set the world No 27 allegedly said in Russian: 'I am f**king sick of giving careers back to disabled people.'

It was Thiem's first Grand Slam win in more than two years after struggling with a serious wrist injury.

Bublik has also lost to Stan Wawrinka and Gael Monfils recently after they had both returned from absences due to injury.

Kyrgios, who is struggling with his injuries at the moment, was quick to comment on the incident, writing on X (previously known as Twitter): 'This is low-key horrible.'

Tennis journalist and commentator Jose Morgado also slammed Bublik on Twitter, accusing him of 'a tremendous lack of respect'.

When Thiem was questioned about the apparent insult during his press conference, he wondered if the translation was correct.

Informed by a reporter that it was, Thiem disapproved of the remark.

'It's a tough saying,' Thiem said.

'Something you shouldn't say in general, out of respect. Maybe he didn't mean it that way.'

Tennis fans were also outraged by the alleged sled from Bublik, who has a history of controversial moments on the court.

One fan wrote on Twitter: 'I sometimes can't understand how he manages to escape the criticism and fines that Nick Kyrgios gets. His on-court behavior is just as bad as NK, and he's done and said so many offensive things?'

Another replied: 'Bublik says s**ty things practically every other match. This is disgusting even by his standards though.

Bublik created headlines earlier in the year when he lost all composure and smashed three of his racquets in a stunning on-court meltdown in Montpellier.

Thiem will next face young American Ben Shelton in the second round of the US Open.