Serena Williams says Tennys Sandgren should apologise to an "entire group of people" following the 26-year-old's controversial social media activity.

Sandgren's surprise showing at the Australian Open, where he reached the quarter-finals having never previously gone beyond the first round of a slam, has been overshadowed after he was questioned over links to the alt-right movement on Twitter.

The 26-year-old defended himself on Monday, saying he "finds some of the content interesting" but insisting that he is not an alt-right sympathiser.

Sandgren then took aim at the media following his last-eight defeat to Hyeon Chung, accusing reporters of "perpetuating propaganda machines".

American star Williams, who has 23 grand slam singles titles, tweeted before Sandgren's defeat, merely saying: "Turns channel."

And she later posted directly to her compatriot, writing: "@TennysSandgren I don't need or want one. But there is an entire group of people that deserves an apology.

"I can't look at my daughter and tell her I sat back and was quiet. No! She will know how to stand up for herself and others- through my example."

Accompanying the post was a picture carrying the quote: "Maturity is being able to apologize and admit when you're wrong because you know that your mistakes don't define you."