Naomi Osaka has revealed what Serena Williams said whilst comforting the 20-year-old after her decisive and shock US Open win.

The Japanese star defeated her childhood hero 6-2 6-4 in a highly controversial match which stunned tennis fans around the world.

Williams was handed a code violation for coaching early in the second set when Patrick Mouratoglou appeared to make hand signals to the 23-time Grand Slam champion from the player’s box.

The 36-year-old then began ranting at umpire Carols Ramos and stormed over to his chair shouting: “I don’t cheat to win! I’d rather loose….I’m just letting you know!”

A furious Williams then launched a series of verbal attacks on Ramos, demanding an apology for his call on the coaching violation, accusing the well-respected and experienced umpire of being “a liar and a thief” for stealing a point away from her.

Williams again informed Ramos that she is not a cheat and did not receive coaching from Mouratoglou, even adding that she had a daughter so would never do that.

“You owe me and apology,” she said. “I have NEVER cheated in my life, I have a daughter and I stand for what is right and I have NEVER cheated.”

Ramos sent Williams into a further rage when he handed the superstar a game penalty for verbal abuse.

Williams' behaviour somewhat overshadowed Osaka's achievements as she held her composure to become Japan’s first ever Grand Slam champion.

The 20-year-old became clearly emotional after winning, and pulled her visor over her face as Williams put a comforting arm around her.

Ellen Show host Ellen Degeneres asked Osaka what Williams said to her after the match, and Osaka replied that Williams was very reassuring.

“She said she was proud of me and that I should know that the crowd wasn’t booing at me, so I was really happy she said that,” Osaka said, per the Sydney Morning Herald.

“At that time I did kind of think they were booing at me, because I couldn’t tell what was going on because it was just so loud in there, it was a little bit stressful.”

While the match was full of drama and controversy, Osaka told Degeneres that she didn’t even know what had happened exactly because she was taught as a child to not look when an opponent shows anger on court.

“You’re just told to turn around and try to focus so I tried to do that,” Osaka said.

“But in my mind, I really wanted to know what was going on.”

Osaka had thrust herself into the limelight earlier in the year with a stunning victory over Daria Kasatkina in the final at Indian Wells claiming her first WTA title.

The Japanese star not only walked off court last Saturday with the prestige that comes with winning the nation’s most famous tennis tournament, she also got a check for $3.8 million.

Her new fame and fortune is sure to boost her already rising status within the tennis world.