For the second straight year, Simona Halep's US Open will end before it can really begin. Kaia Kanepi defeated Halep 6-2, 6-4 on Monday in the first round in New York in the first match at the new Louis Armstrong Stadium. It's the second straight year that Halep has lost in the first round of the US Open, and the first time in the professional era that a No. 1 seed has lost in the first round of the tournament.

2018 saw Halep make two Grand Slam finals, including her first career Grand Slam win at the French Open, but it ended in a whimper with Kanepi breaking Halep's serve five times.

Halep will retain her No. 1 status, having entered the US Open over 2,000 points ahead of Caroline Wozniacki in the WTA standings. However, her struggles early in tournaments remain a point of concern. She's now lost in the first round of 12 majors, including five first-round exits since her first final appearance in 2014. About a month after winning the French Open this year, Halep lost in the third round of Wimbledon to Hsieh Su-wei.

Halep pulled out of the Connecticut Open last week ahead of a first-round matchup with Camila Giorgi with an Achilles injury, saying that "I really wanted to play it and I saw that many fans bought tickets to see me here. But I feel very sore with my Achilles, and I need some rest. I had so many matches in the last two weeks, so it's tough. See you next year, and all the best," via The Independent.

Halep's exit is a huge boost for Serena Williams, who is chasing her 24th Grand Slam, and was in the same section of the women's draw as Halep. This upset definitely makes the route to the quarterfinals look more manageable, although dark horses like Kanepi and Giorgi can't be downplayed in any given match, especially if Kanepi plays like she did against Halep.

For Halep, even though she'll keep her No. 1 spot, it can't be the way that she expected her year to end. 2018 is still the best season of Halep's career -- she made two finals after all -- but third and first-round exits to close out the season will leave a sour taste in her mouth heading to Melbourne to open the next Grand Slam season.

Halep is only 26, so she'll hang around the top of the standings for a while. The next thing she'll undoubtedly want to focus on is consistently making it deep into Grand Slam play.

Elsewhere in the women's bracket, Garbine Muguruza looked utterly dominant in her first match against Zhang Shuai. Elise Mertens got a scare from Kurumi Nara in a three-set battle. Elina Svitolina took a three-set bout against Sachia Vickery, and Camila Giorgi continued her excellent season with a sweep over Whitney Osuigwe.

On the men's side of things, John Isner won in straight sets against fellow countryman Bradley Klahn. Grigor Dimitrov continued his nightmare 2018 with his second straight first-round knockout against Stan Wawrinka. Kyle Edmund was also upset by Paolo Lorenzi in a four-set match. Jack Sock and Milos Raonic, meanwhile, lived up to their seedings with wins. Andy Murray also made his grand return to Grand Slam play, defeating James Duckworth in four sets in his first match in a Grand Slam in over a year.

Here's a rundown of the day's scores.

Men's bracket scores

  • Stan Wawrinka defeats No. 8 Grigor Dimitrov 6-3, 6-2, 7-5
  • No. 11 John Isner defeats Bradley Klahn 7-6, 6-3, 6-4
  • Paolo Lorenzi defeats No. 16 Kyle Edmund 4-6, 6-4, 7-5, 6-1
  • Jason Kubler defeats No. 19 Roberto Bautista Agut 6-3, 6-3, 6-4
  • No. 18 Jack Sock defeats Guido Andreozzi 6-0, 7-6, 6-2
  • No. 25 Milos Raonic defeats Carlos Berlocq 7-6, 6-4, 1-6, 6-3
  • No. 27 Karen Khachenov defeats Albert Ramos Vinolas 6-3, 6-2, 6-3
  • Andy Murray defeats James Duckworth 6-7, 6-3, 7-5, 6-3
  • Lorenzo Sonego defeats Gilles Mueller 7-6, 6-7, 5-7, 7-6, 6-2
  • Ugo Humbert defeats Collin Altamirano 6-3, 7-6, 6-3
  • Guido Pella defeats Casper Ruud 6-4, 6-4, 6-1
  • Gilles Simon defeats Lloyd Harris 6-2, 6-2, 6-3
  • Nicolas Jarry defeats Peter Gojowczyk 7-6, 4-6, 6-1, 7-5

Women's bracket scores

  • Kaia Kanepi defeats No. 1 Simona Halep 6-2, 6-4
  • No. 7 Elina Svitolina defeats Sachia Vickery 6-3, 1-6, 6-1
  • No. 12 Garbine Muguruza defeats Zhang Shuai 6-3, 6-0
  • No. 15 Elise Mertens defeats Kurumi Nara 6-2, 6-7, 7-5
  • No. 18 Ashleigh Barty defeats Ons Jabeur 6-1, 6-3
  • Rebecca Peterson defeats No. 27 Anastasia Pavlyuchenkova
  • Wang Quiang defeats No. 31 Maria Rybarikova 6-2, 6-2
  • Camila Giorgi defeats Whitney Osuigwe 6-4, 6-1
  • Tatjana Maria defeats Agnieszka Radwanska 6-3, 6-3
  • Jil Teichmann defeats Dalila Jakupovic 6-3, 6-0
  • Irina-Camilia Begu defeats Jennifer Brady 6-3, 6-3
  • Ekaterina Makarova defeats Heather Watson 6-1, 3-6, 6-3
  • Vera Lapko defeats Kateryna Bondarenko 6-3, 6-3
  • Vania King defeats Natalia Vikhlyantseva 7-6, 6-3