The 2006 champion had looked set to win in straight sets when she led 4-1 in the second only to let it slip but recovered to clinch a dramatic victory after two hours and 44 minutes.

Sharapova, not known for public displays of emotion, dropped to her knees and then sat and sobbed on her chair, her face a picture of disbelief.

Addressing the crowd, the 30-year-old said: "I just thought this was another day, another opportunity, another match but this was so much more and I didn't want to think about it. You sometimes wonder why you put in all the work and this is exactly why."

So what had she learned from a match that felt more like a final than an opening-night encounter?

"That behind this black dress with Swarovski crystals this girl has a lot of grit and she is not going anywhere," said Sharapova.

Snubbed by the French Open and then injured for Wimbledon, where she would have had to fight her way through qualifying, Sharapova was controversially given a wild card in New York.

If there were dissenters among the crowd, they kept their thoughts to themselves as the Russian strode onto court in her sparkly dress with cheers ringing in her ears.

She had beaten Halep in all six of their previous meetings but this time came in as the firm underdog having played just four tournaments since the end of her suspension in April.

Injuries have been her nemesis and she had managed only one match in more than three months.

But from the start it was clear Sharapova was fit and in the mood to write headlines on the court at last.

She said: "When the draw came out, I knew what a difficult match-up this would be, how much excitement was behind this match-up and the hype.

"It almost seemed like I had no right to win this match today. And I somehow did. I think that is what I'm most proud of.

"I definitely have to value the feeling that I have now. I think that's important. I can't take that for granted. I can't take the level for granted. I can't take my emotions for granted.

"This is a big win for me, and I will enjoy it, then move on to the next one."

Halep had been one of the critics of the decision to give Sharapova a wild card for her comeback tournament in Stuttgart, although she took a more conciliatory approach here.

Sharapova needed no extra motivation back on the stage she loves best and set about pummelling the Halep serve.

The cavernous arena almost swallowed up the famous Sharapova shriek as both the winner and error counts rocketed (the Russian's final tally of 60 winners and 64 errors compared to 15 and 14 for Halep).

Sharapova's lack of match toughness showed in the second set as she lost five straight games having had opportunities in all of them but she regrouped at the start of the third set and did not allow the same thing to happen again.

This was undoubtedly a crushing loss for Halep, although it is not impossible she could yet end the tournament as world number one.

"I'm sad, of course, losing this match," said the second seed. "But I think I gave everything I had. She was better. She was serving better. My serve was very bad today.

"It was not like a first-round match. But this is the draw. I cannot say anything else. It's the luck. It was a good match. It was good for the fans."

Sharapova moves through to a second-round meeting with Hungary's Timea Babos on Wednesday.