Roger Federer believes fellow tennis great Serena Williams went "too far" in her outburst during September's US Open final.

Williams called an umpire a liar and a thief and accused him of being sexist on her way to defeat against Naomi Osaka.

Federer told the Sunday Times it could have been handled differently.

"I feel like Serena should have walked away," Federer said.

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"She did, but she went too far. She should have walked earlier."

He did, however, admit that Williams' actions were a "little bit excusable".

"The umpire maybe should not have pushed her there. It's unfortunate, but an incredible case study."