Simona Halep made a quick-as-can-be exit from the U.S. Open on Monday, becoming the first No. 1-seeded woman to lose her opening match at the Grand Slam tournament in the half-century of the professional era.

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.

It was the first match at the rebuilt Louis Armstrong Stadium, which now has a retractable roof, and what a way to get things started. That cover was not needed to protect from rain on Day 1 at the year's last major tournament — although some protection from the bright sun and its 90-degree (33-degree Celsius) heat might have been appreciated.

Since professionals were admitted to the Grand Slam tournaments in 1968, only five times did women who were seeded No. 1 lost their opening match at a major — and never before had it happened at the U.S. Open. It happened twice to Martina Hingis and once to Steffi Graf at Wimbledon, once to Angelique Kerber at the French Open and once to Virginia Ruzici at the Australian Open.

Halep won the French Open in June for her first Grand Slam title and is assured of remaining at No. 1 after the U.S. Open, even with her first-round departure. She got off to a slow start at Roland Garros this year, too, dropping her opening set, also by a 6-2 score, but ended up pulling out the victory there and adding six more to lift the trophy.

There would be no such turnaround for her against Kanepi, a big hitter who dictated the points to claim her second career win against a top-ranked player. Kanepi has shown the occasional ability to grab significant results, including a run to the quarterfinals at Flushing Meadows a year ago — when Halep also lost in the first round.

Halep's 2017 defeat came against five-time major champion Maria Sharapova, though. Kanepi is simply not that caliber of player.

On this day, though, Kanepi took charge of baseline exchanges, compiling a 26-9 edge in winners, 14 on her favored forehand side alone. Wearing two strips of athletic tape on her left shoulder, the right-handed Kanepi also had far more unforced errors, 28-9, but that high-risk, high-reward style ultimately paid off.

"Staying aggressive," Kanepi said, "all the time."

Early in the second set, on the way to falling behind by two breaks at 3-0, Halep slammed her racket twice, drawing a warning for a code violation from the chair umpire.

Eventually, Halep got going a bit, taking advantage of Kanepi's mistakes to break back twice and get to 4-all in that set.

But Kanepi ended a 14-stroke exchange with a cross-court forehand volley winner to break right back, then served out the victory.

A long list of returns was scheduled for later Monday, including past champions Serena Williams, Andy Murray and Stan Wawrinka on the court after missing last year's U.S. Open.

In addition, defending champions Rafael Nadal and Sloane Stephens were slated to play.

Williams was to face Magda Linette in Arthur Ashe Stadium to start the night session, followed by Nadal against David Ferrer in a rematch of the 2013 French Open final.