World number one Simona Halep was upset 6-2 6-4 by unseeded Estonian Kaia Kanepi in the U.S. Open first round on Monday, the second consecutive year the Romanian has lost her opening match at Flushing Meadows.

Playing in the first match on the newly renovated Louis Armstrong Stadium court, the French Open champion failed to sustain any kind of service rhythm and her usually stout defence proved no match for Kanepi's bruising groundstrokes and deft drop shots.

"I have always loved being in New York," the 33-year-old Kanepi said.

"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."

The only consolation for Halep is that she has amassed enough ranking points to remain world number one after the two-week tournament in New York.

Kanepi earned the first break when she launched a back-hand lob over Halep's head to jump out to a 2-1 lead.

Kanepi broke again in the fifth game and never looked back as Halep won just 59 percent of her first-serve points compared to 80 percent for Kanepi in the set.

Frustration for Halep boiled over in the second game of the second set when she smashed her racket on the court, earning a racket abuse warning from the chair umpire.

Halep recovered and it appeared a comeback was on when she broke Kanepi to love to level the second set at 4-4.

But Kanepi completed the upset when Halep sent a service return long on match point.

It was the sixth time in the Open era that the number one seed has lost in the first round of a Grand Slam tournament and the first at the U.S. Open.

Halep had looked ready for a long run in New York after she won the Rogers Cup this month and narrowly missed out on back-to-back titles after losing to Kiki Bertens in the Cincinnati final.