Unseeded and ranked 81st, Williams won for her eighth and most improbable Grand Slam title Saturday, beating Sharapova 6-1, 6-2.
"It was an awesome win, because I had so many critics. So many people ... saying negative things," Williams said. "Saying I wasn't fit, when I felt that I was really fit, and I could last three sets.
"It's always like, tell me no and I'll show you that I can do it. I get the greatest satisfaction just holding up the Grand Slam trophy and proving everyone wrong."
Only the second unseeded woman to win the Australian title in the Open era, Williams came to Melbourne Park with only three matches at a low-key warmup tournament after missing most of last season because of a knee injury.
After entering the tournament ranked No. 81, Serena takes home the trophy. (Getty Images)
She will jump to No. 14 in the world next week after capturing her first title in 15 tournaments since her second Australian Open victory in 2005.
Despite doubts about her fitness, Williams thought she was in great shape.
"I have been working really hard and haven't felt like my results have been showing," she said. "And finally it just all came together, especially in the final match."
The victory was the most dominating win in a completed championship match at Melbourne Park since Steffi Graf beat Arantxa Sanchez-Vicario 6-0, 6-2 in 1994.
"This definitely goes down as one of my good matches. I'm insatiable," Williams said. "I think I could have done better. That's just my character."
She finished off the top-seeded Sharapova with a backhand winner, then threw her racket in the air and dropped onto her back on the court.
The 25-year-old American, also the 2003 champion, got up and skipped over to the side of the court, swapping high-fives with mother Oracene Price and other people in the stands before jogging back onto court with her arms up in triumph.
Williams dedicated the title to her half-sister, Yetunde Price, who was killed in a 2003 shooting.