Nick Kyrgios was booed off the court after retiring midway through a second round clash against American Tennys Sandgren at the Citi Open in Washington.

Kyrgios then had a post-match press conference cut short, prompting New York Times tennis journalist Ben Rothenberg to say on Twitter he was fighting “obvious off-court mental struggles”.

Press was cut off by ATP after less than 2 min, but Kyrgios said physical struggles hurt his prep. Also obvious off court mental struggles.

— Ben Rothenberg (@BenRothenberg) August 3, 2017

The Australian hit the headlines last week when he was spotted playing basketball in Canberra just days after pulling out of the Atlanta Open due to a hip injury.

And he withdrew from his third match in a row on the tennis tour after falling a set down to Sandgren.

The unseeded American led 6-3 3-0 when Kyrgios pulled the pin, allegedly citing a shoulder issue, to the disgust of some sections of the crowd.

https://twitter.com/KelynSoong/status/892954794368872449

Sandgren, who was ranked as low as 330th in the world last year, is now the world’s 106th-best player.

But he made light work of Kyrgios, racing through the first set-and-a-half in just 38 minutes.

In that time, he won 17 of 19 points on his first serve and converted three of his six break points to claim the early ascendancy.

Kyrgios could not rely on his big serve as he usually does, mixing five aces with five double faults.

He also landed only 49 per cent of first serves in an uncharacteristically poor effort.

The world number-20 only won four points on Sandgren’s serve during the encounter.

His decision to retire was met with booing by the crowd at the ATP 500 event and meant Kyrgios has not completed a match since the French Open.

Commentating on US television, Jim Courier said that during the match he could lip-read Kyrgios telling his mother, who was in the crowd: “I don’t want to play.”

Kyrgios withdrew from his only match at the Aegon Championships in June after losing the first set to American Donald Young, while his only clash at Wimbledon was also cut short.

Frenchman Pierre-Hugues Herbert led two sets to love at the All-England Club when Kyrgios retired.

He has won just three ATP Tour matches since reaching the semi-finals of the Miami Open, a tournament in which he beat Ivo Karlovic, David Goffin and Alexander Zverev.