World No. 1 Simona Halep fell in the first round at the 2018 U.S. Open on Monday as she was defeated 6-2, 6-4 by Estonia's Kaia Kanepi.

The French Open champion looked to have turned the momentum in her favour when she levelled from a double break down to 4-4 in the second set. But she was then broken again before world No. 44 Kanepi secured a momentous triumph.

In the men's draw, eighth seed Grigor Dimitrov was the biggest casualty in Monday's opening session as he lost 6-3, 6-2, 7-5 to three-time Grand Slam champion Stan Wawrinka.

Elsewhere, Andy Murray came through in four sets against James Duckworth, but his fellow Brit, Kyle Edmund, was downed by Italy's Paolo Lorenzi.

Meanwhile, defending women's champion Sloane Stephens and two-time winner Venus Williams both prevailed in their opening round matches.

Here is a selection of results from the opening day of action in New York:

Men's Singles

Stan Wawrinka bt. (8) Grigor Dimitrov: 6-3, 6-2, 7-5

Andy Murray bt. James Duckworth: 6-7 (5), 6-3, 7-5, 6-3

(11) John Isner bt. Bradley Klahn: 7-6 (3), 6-3, 6-4

(25) Milos Raonic bt. Carlos Berlocq: 7-6 (4), 6-4, 1-6, 6-3

Women's Singles

(16) Venus Williams bt. Svetlana Kuznetsova: 6-3, 5-7, 6-3

Kaia Kanepi bt. (1) Simona Halep: 6-2, 6-4

(3) Sloane Stephens bt. Evgeniya Rodina: 6-1, 7-5

(12) Garbine Muguruza bt. Shuai Zhang: 6-3, 6-0 

Full results and schedule available via the U.S. Open's official website.

Per Sports Illustrated, Halep's predecessors as top seed at the U.S. Open had all enjoyed first-round success, and she now holds an ignominious record:

The Romanian went into the U.S. Open as one of the favourites, but she was outclassed by Kanepi at the Louis Armstrong Stadium as she was broken on five occasions.

Halep only had nine winners in the match compared to Kanepi's 26, and she won just six points behind her second serve.

She was blown away in the opener, not forcing a single break point on Kanepi's serve and losing her own twice.

The underdog then secured a double break in the second before Halep rallied back to 4-4.

With the momentum back in her favour, it looked as though the 2015 semi-finalist at Flushing Meadows might save herself from a shock exit.

But Kanepi showed huge composure and skill to break again and then serve for the match, and Halep went long with a forehand on match point.

There were no such problems for third seed Stephens, whose 6-1, 7-5 defeat of Evgeniya Rodina lasted just 76 minutes.

Williams had to work much harder for her victory over Svetlana Kuznetsova. The two multiple Grand Slam champions put on a show for those in Arthur Ashe Stadium, exchanging 14 breaks of serve in a match that ran to almost three hours.

In the men's draw, Murray played out his first best-of-five-set match in 14 months and eventually came through in three hours, 17 minutes after losing the first set in a tiebreak.

Every victory will be precious for the Scot given he is only gradually returning to action following a serious hip injury, and he was clearly delighted with his gritty win in intense heat, per Team GB:

Wawrinka is another star who has been struggling with injury in the last year, but he looked mightily impressive as he beat Dimitrov in the first round for the second Grand Slam in succession:

The Bulgarian had his chances to make more of a contest of the clash, forcing eight break points on his opponent's serve.

However, he took only one of them in the third set, while Wawrinka claimed five from 14.

The Swiss also had more winners (30 to Dimtrov's 29), fewer unforced errors (37 to 42), and a better first serve percentage (61 to 52).

It was a match that swung on fine margins and Wawrinka won the crucial moments. He will now face 20-year-old Frenchman Ugo Humbert in the second round.