The Romanian, the world number one who broke her major duck by winning the French Open this year, was stunned 6-2, 6-4 by Estonia's Kaia Kanepi.

Twenty-six-year-old Halep became the first top seed to lose her opening match on Flushing Meadows' hard courts in the open era.

"I have always loved being in New York," Kanepi, who has made the US Open last eight twice, said.

Halep, who pulled out of last week's Connecticut Open with an Achilles problem, admitted she looked "lost" despite winning the recent Cincinnati Masters. 

There were no such problems in the women's draw for home favourite Serena Williams who roared past Poland's Magda Linette.

The six-time winner and 23-time Grand Slam champion, seeded 17th, powered home 6-4, 6-0.

Her sister Venus also advanced in a trickier clash, with the two-time winner and 16th seed needing three sets to beat Russian wildcard Svetlana Kuznetsova 6-3, 5-7, 6-3.

Third seed and defending champion Sloane Stephens also won on a good day for the high profile Americans, 6-1, 7-5 against Russia's Evgeniya Rodina.

In the men's draw, top seed and defending champion Rafael Nadal came through against compatriot David Ferrer when his opponent pulled out injured.

Nadal, a three-time winner who has 21 Grand Slam titles in all, was 6-3, 3-4 in front when Ferrer had to withdraw with a calf problem,

It spoilt Ferrer's last Grand Slam tournament before his retirement.

In other matches, Britain's former champion Andy Murray beat James Duckworth of Australia 6-7, 6-3, 7-5, 6-3 on his Grand Slam return follow a long-term hip injury.

Switzerland's Stan Wawrinka, the 2016 winner given a wildcard this time around, knocked out Bulgaria's eighth seed Grigor Dimitrov in a blockbuster first round clash, 6-3, 6-2, 7-5.