Roger Federer has said that Serena Williams "went too far" in her on-court criticism of umpire Carlos Ramos during the US Open final.
Williams received a code violation for coaching during the US Open final, which sparked a cascade of incidents leading to the 23-time grand slam champion being penalised a game on her way to a straight sets defeat to the previously-unheralded Naomi Osaka.
In an interview with Britain's The Sunday Times, the 20-time Grand Slam champion said that Williams went too far with her on-court protestations.
"I feel like Serena should have walked away," Federer said.
"She did, but she went too far. She should have walked earlier."
Federer did caveat his comments by saying her reaction was understandable in the context of the situation she was placed in.
"It's a little bit excusable. The umpire maybe should not have pushed her there.
"It's unfortunate, but an incredible case study."
The incident polarised the tennis world, but Federer said the conversation that arose from the controversy was a good thing for the sport.
"Anything that's good for society and gets the ball rolling, I'm all for it." Federer said.
"I dove into the situation with Serena on so many levels, to understand the umpire, Osaka, Serena, the crowd."
Williams's outburst in the second set of the US Open final overshadowed Naomi Osaka's 6-2, 6-4 victory, in what was the 20-year-old's maiden appearance in a Grand Slam final.
Williams first received a code violation for coaching at the start of the second set, an accusation she vehemently denied in an emotional exchange with umpire Ramos.
From that point, the situation escalated dramatically.
At the end of the fifth game of the second set, Williams destroyed her racquet on the court, receiving a second violation which carried a point penalty.
Osaka went on to hold that game to love and broke Williams in the following game to go 4-3 up having won the first set.
At the change of ends, Williams was penalised a third time for calling Ramos a "thief" for "stealing a point" from her, resulting in a game penalty to fall further behind at 3-5.
The partisan crowd at Flushing Meadows were incensed, booing almost every point, eventually reducing Osaka — who become the first Japanese woman to win a grand slam — to tears.
Williams, who later claimed that the penalties against her were "sexist," was fined $US17,000 ($23,608) by the United States Tennis Association for the code violations.