Naomi Osaka won the final 6-2, 6-4 to become Japan's first ever Grand Slam singles champion. It also delayed Williams's bid for a record-equalling 24th major title.
However, the final was overshadowed by the American's angry and tear-filled meltdown in the second set.
She was handed a code violation for coaching, a penalty point for racquet abuse and a game penalty for calling umpire Carlos Ramos a "liar and a thief" and insisting "you owe me an apology".
Williams insisted that her coach Patrick Mouratoglou had not been coaching her even though the Frenchman told ESPN that he had and that all coaches do it.
"I just texted Patrick, like, What is he talking about? Because we don't have signals. We have never discussed signals," said Williams.
Williams said women players are treated differently to their male counterparts in the sport.
Serena Williams of the United States argues with referee Brian Earley.
"I've seen other men call other umpires several things. I'm here fighting for women's rights and for women's equality," said the 36-year-old.
"For me to say 'thief' and for him to take a game, it made me feel like it was a sexist remark.
"He's never taken a game from a man because they said 'thief'. For me it blows my mind. But I'm going to continue to fight for women."
Williams made reference to the incident last week when French player Alize Cornet was warned for removing her shirt on court.
Cornet was accused of "unsportsmanlike behaviour".
"Cornet should be able to take off her shirt without getting a fine. This is outrageous," said Williams warming to her theme.
"I just feel like the fact that I have to go through this is just an example for the next person that has emotions, and that want to express themselves, and want to be a strong woman.
"They're going to be allowed to do that because of today. Maybe it didn't work out for me, but it's going to work out for the next person."