Williams received a code violation for coaching, a penalty point for racquet abuse and a game penalty for calling the umpire a "liar" and a "thief" during her defeat by Naomi Osaka.

Federer said the incident could have been handled differently.

"I feel like Serena should have walked away," he told the Sunday Times.

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

Federer did, however, say Williams' actions were "a little bit excusable".

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

What happened in New York?

Osaka's 6-2 6-4 victory over Williams on 9 September was overshadowed by the American's outbursts during the second set.

Williams, 37, was given a first code violation after umpire Carlos Ramos judged a gesture from coach Patrick Mouratoglou to be coaching.

The American - a 23-time Grand Slam singles champion - told Ramos she would "never cheat to win and would rather lose". Mouratoglou later admitted he had been coaching from the box.

Williams received another code violation for smashing her racquet, leading to Ramos docking her a point.

When the game finished, she called him "a liar" and "a thief". That led to Ramos docking her the next game - leaving Osaka needing just one for victory.

Williams later said it was "sexist" to have been penalised a game.

She was fined $17,000 (£13,100) by the United States Tennis Association for the code violations.

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'What was the problem with the catsuit?'

In the Sunday Times interview, Federer also addressed the subject of female players' on-court attire.

It was announced in September that Williams would be banned from wearing a black catsuit at future French Opens, with the tournament set to introduce a stricter dress code.

She wore the outfit at the tournament in May, and said it made her "feel like a superhero".

At the US Open, meanwhile, France's Alize Cornet was given a code violation for changing her shirt on court.

"What was the problem with taking the shirt off or the catsuit?" said Federer, a 20-time Grand Slam singles champion.

"Serena has worn crazier stuff in the past. Guys have worn crazier stuff. For me it was all a bit of nonsense.

"I was totally on the women's side. Leave them alone."