Despite being in her first Grand Slam final, Osaka showed confidence and raced into a 5-1 lead in the first set.
Osaka won the set 6-2 and both players made an even start to the second set butit exploded when umpire Carlos Ramos gave Williams a code violation for coaching from her box.
Williams then confronted the umpire claiming she doesn’t cheat but after dropping serve at 4-3 she smashed her racket leading to a point penalty.
The 36-year-old then called the umpire a thief which led to a game penalty and that allowed Osaka to serve out the match and win her first Grand Slam.
Williams then received another for a racquet smash at 3-2 in the second set, leading to Ramos docking her a point as Osaka started at 15-0 in the following game.
The American was furious, walking up to Ramos, shouting and pointing at him as the crowd started booing in support of the former world number one.
The drama continued as the atmosphere in Arthur Ashe Stadium became more toxic.
Williams refused to let the issue slide and launched an extraordinary rant at Ramos at the changeover, with Osaka leading 4-3.
“You are a liar. You will never be on a court of mine as long as you live. When are you going to give me my apology? Say you are sorry,” she told the Portuguese.
However, Osaka said: “I know everyone was cheering for her and I’m sorry it had to end like this. I just want to say thank you for watching the match.
“It was always my dream to play Serena in the US Open finals. I’m really glad I was able to do that.”