The reigning French Open champion became the first top-seeded woman to lose in the first round of the U.S. Open when she fell to No. 44 Kaia Kanepi, 6-2, 6-4, in the tournament's first match in a newly renovated Louis Armstrong Stadium.
It was the second consecutive year the Romanian French Open champion exited in the U.S. Open first round.
Halep simply was overwhelmed by the power-based game of 44th-ranked Kaia Kanepi of Estonia 6-2, 6-4 in a match that was stunningly lopsided and lasted all of 76 minutes.
"I thought I just have to be aggressive and try to stay calm", said Kanepi, whose run to the quarter-finals previous year is just one of her six trips to the last eight in majors.
No.7 seed Elina Svitolina powered into the second round of the US Open thanks to a 6-3, 1-6, 6-0 victory over Sachia Vickery who broke in the very first game before Svitolina claimed five straight games.
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. Kanepi, who came within two points of victory before losing their lone encounter at 2014 Doha, was asked what was the secret to her win over Halep, BBC reported.
Halep was off to another slow start at a Grand Slam tournament.
The Estonian fired 26 winners to Halep's nine, cutting the rallies short and remorselessly punishing Halep's second serve.
Rated as the joint favourite with Serena Williams heading into the event, Halep came unstuck to Kanepi - who played a brilliant match - and made an unwanted piece of history as she became the first women's top seed to lose in the opening round in NY in the Open Era.
"I love opening for Serena Williams", Grammy-winning singer-songwriter Kelly Clarkson told the crowd before she launched into her anthem Stronger. "I couldn't play better, so that's it". I'm not complaining. I just mean that I don't really feel my game at 100% when I step on the court here.