Simona Halep of Romania hits a return during the Women's Singles first round match against Kaia Kanepi of Estonia at the 2018 US Open tennis championships in New York, the United States, Aug. 27, 2018. Simona Halep lost 0-2. (Xinhua/Wang Ying)

NEW YORK, Aug. 27 (Xinhua) -- Top seed Simona Halep's U.S. Open campaign was cut short in the first round after losing to Kaia Kanepi, who needed just two sets to score the first big upset of the season's last Grand Slam at 6-2, 6-4 on Monday.

The Romanian has become the first No. 1-seeded woman to lose in U.S. Open first round in the professional era.

Halep also lost in the first round at last year's U.S. Open to 2006 champion Maria Sharapova, but she was not world No. 1 at that time.

"It's always about the nerves," said Halep. "Even when you are there in the top, you feel the same nerves. You are human."

"I'm a quiet person, so maybe I like the smaller places," she added.

Halep, who had claimed her first Grand Slam title in Paris in June, came into the U.S. Open 2018 flying high after a string of big results during the U.S. Open series, lifting the title in Montreal and reaching the final in Cincinnati.

Kanepi, ranked No. 44 in the world, reached the quarterfinals here last year -- her best result at any Grand Slam -- but has struggled to find her form after suffering a series of injuries.

But it was the 33-year-old Estonian who was the fresher player on Monday, playing the first match of the day against Halep on the brand new Louis Armstrong Stadium. She fired 26 winners compared to just nine from Halep.

It was the first match at the rebuilt Louis Armstrong Stadium, which now has about 14,000 seats and a retractable roof. However, Day 1 witnessed 33-degree Celsius heat instead of rain.

"The courts suit my game, and I love being in New York. I like the city," said Kanepi. "I like the weather: humid and hot."

"Yesterday I thought I have beaten just once the world number one, today I had another chance," said Kanepi, who had beaten then No. 1 Caroline Wozniacki in Tokyo in 2011.

"I thought, I just have to be aggressive and try to stay calm during the game," Kanepi said.

Coming up next for Kanepi will be Swiss player Jil Teichmann, who made a successful main draw debut winning 6-3, 6-0 against Dalila Jakupovic of Slovenia.