The French Open champion, 26, arrived at Flushing Meadows as one of the hot favourites and declared herself free of pressure after breaking her Grand Slam duck at Roland Garros.
But it did not appear that way against Estonia's Kaia Kanepi, one of the biggest hitters in the game, who blasted her way to a 6-2 6-4 victory.
Kanepi, 33, has an impressive Grand Slam pedigree having reached six quarter-finals, including here last year, when she progressed to the last eight as a qualifier after injury problems had seen her drop down the rankings.
Nothing in her results this season suggested she was about to derail a confident Halep but the Romanian, who lost to Maria Sharapova at the same stage last year, knew what a potentially tricky draw it was and so it proved.
Kanepi tightened up in the second set, seeing a 3-0 lead wiped out at 4-4, but she kept swinging for the lines and, after breaking Halep for the fifth time, confidently served out one of the wins of her life.
The good news for Halep is she is guaranteed to hold onto the world number one ranking having established a solid lead over her rivals.
Speaking after her emphatic win, Kanepi said: "I have always loved being in New York.
"I like the atmosphere. I like being here. I love the courts and the climate, and I think that the courts suit my game really well."
She will now play Jil Teichmann in the second round on Wednesday.
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