Holger Rune shared a brutally honest statement to social media after he suffered an upset loss at the US Open.

The 15th seed was stunned by Brandon Nakashima in the first round as the American earned a routine 6-2 6-1 6-4 victory.

Rune has now lambasted his own performance and hinted that his poor quality was the reason for Nakashima’s win, though he said he meant “no offence” to his opponent.

Rune didn’t even last two hours on the court in his opening match at the US Open. The 21-year-old hit 30 unforced errors and failed to convert his six break point opportunities in a lacklustre showing in the last Grand Slam of the season.

After having a day to digest his defeat, the former world No. 4 has now made it clear that he wasn’t satisfied with his game. “Big bummer yesterday,” Rune wrote on social media, adding a heartbroken emoji.

“The best support, lots of happy tennis kids, perfect weather, perfect court and tournament and I played extremely bad tennis.”

The four-time title winner reeled off some of his worst match statistics, indicating that he thought his defeat was the result of his own poor play and not a high-quality statement from Nakashima. But Rune tried to soften the blow to his rival.

He added: “No offence to my opponent, but I managed only 39% !!first serves , tons of unforced errors, this is not what I or anyone should expect.”

And Rune shared a defiant statement, vowing not to disappoint his supporters. “I need to look into everything now. This is not satisfying for me or my fans. I will not let you down anymore,” the Dane concluded.