An enthralling first-round match at Arthur Ashe Stadium saw the 19-year-old Watson eventually go down a 3-6 7-5 6-3 victory after two hours and 34 minutes.
And Watson, a former US Open junior champion, was pleased with the way she played.
She said: "I had a lot of fun today. It's what I want to do. I want to be out there on the big courts playing the top players. It was a really great match, very competitive. We played some really great tennis.
"I had my chances in the second set. You have to create the opportunity but also execute, and I was probably a bit too defensive on those important points.
"But I always believed I could win. There's no point in my going out there if I don't believe that I can win.
"I just wanted to make every ball, make her play every ball, and just fight. Because Maria's a fighter. She's never going to give up. That's what makes her a champion. That's why she's won this tournament before.
"I absolutely loved the atmosphere. I thought everyone was going to be supporting Maria. She's won here before. She's been number one in the world.
"I had a lot of support. I heard some people calling me Cinderella, all sorts of stuff. And just when they were chanting my name, I kind of got the goosebumps. I thought, 'Stay calm, focus on the match'."
Sharapova was effusive in her praise of Watson and rated the contest as one of her toughest first-round clashes.
The Russian said: "No doubt that it was one of the toughest that I've had. To be honest, it's quite unexpected, obviously not really knowing too much about my opponent and not facing her before.
"She just came out and played really smart. There's no doubt that I wasn't playing my best tennis. She was smart in making me hit another ball. I was making so many errors out there. She stuck to her gameplan. She kept grinding.
"For me it was just a matter of trying to be consistent out there and winning not just one, two, but three points in a row.
"She did many things well. She's a great mover on the court. She made me hit a lot of balls. She was really, really steady. She handled pace pretty well."