All-time great Serena Williams has given the thumbs down to compatriot Tennys Sandgren as the unheralded American attempts to reach the semi-finals of the Australian Open.

"Turns channel," the 23-time major winner tweeted as Sandgren took the court in his quarter-final with fellow outsider Hyeon Chung.

The missive is being widely read as a reference to Sandgren's support for the politics of US President Donald Trump.

Williams isn't defending her title at Melbourne Park after the birth of her first child in September.

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As a Jehovah's Witness, the 36-year-old does not vote but has previously expressed support for former president Barack Obama.

She is a UNICEF goodwill ambassador and has supported fundraising efforts for marginalised communities and anti-violence initiatives.

Tennys Sandgren of the United States wins a point in his quarter-final match against Hyeon Chung of South Korea. Photo / Getty
Tennys Sandgren of the United States wins a point in his quarter-final match against Hyeon Chung of South Korea. Photo / Getty

Sandgren's run to the last eight at the Open has brought to light his support for Trump and interest in nationalist and alt-right politics.

His social media activity was labelled racist by a number of tennis fans but the 26-year-old Tennesee native on Monday denied supporting the far-right movement. He did, however, admit finding "some of the content interesting".

He has since deleted all but one of his tweets in response to scrutiny of his political views.

Read more: Tennessee Tennys deletes tweets, denies alt-right views

Among the deleted tweets was one from November 2016 where Sandgren appeared to back the debunked Pizzagate conspiracy linking then-Democratic presidential candidate Hillary Clinton to a supposed child sex abuse ring at a Washington pizzeria.

And earlier this month, he retweeted a video from white nationalist Nicholas Fuentes.

Sandgren was swiftly dumped out of the tournament by Chung.

The 21-year-old world No.58 won 6-4 7-6 (7-5) 6-3 to also become the lowest-ranked man to make the last four of the Open since Marat Safin in 2004.

Chung will play either five-time champion Roger Federer or Czech Tomas Berdych on Friday night for a spot in the final.