What Serena Williams whispered to Naomi Osaka as the crowd booed after her US Open win.

It was an emotional moment for both Serena Williams and Naomi Osaka as the 20-year-old was awarded her US Open trophy on Saturday.

A section of the crowd could be heard booing after the Japan-born player’s 6-2, 6-4 victory, which was marred by Williams’ various run-ins with umpire Carlos Ramos over code violations.

There on the podium, Williams whispered something to her visibly upset opponent. Now, Osaka has shared what that was.

Speaking on The Ellen Degeneres Show, the young Grand Slam champion told Degeneres that her idol had attempted to comfort her.

"She said that she was proud of me and that I should know that the crowd wasn’t booing at me."

It was important for Osaka to hear, because, as she told the TV host, she did actually think the crowd was booing her.

"I couldn’t tell what was going on, because it was just so loud in there. It was a little bit stressful," she said.

Osaka, who has lived in the US for most of her life, said that while on the court she had attempted to ignore her opponent's arguments with the court's referee, turning her back on it at times.

"When you’re little, you’re taught not to look at, like, if your opponent gets angry or anything," Osaka said.

"You’re told to just turn around and try to focus."

However, Osaka said this required some will-power as she "really wanted to know what was going on".

"I couldn’t hear and I was looking away, but I heard a lot of people in the crowd making noises, and I really wanted to turn around, but I didn’t."

Williams was fine $24,000 after calling Ramos both a "thief" and a "liar" during the match when he sanctioned her for "coaching".