The U.S. Open, the year's fourth and final Grand Slam tournament, got underway Monday at Flushing Meadows in New York with men's and women's singles play.

Last year, there were no fans in the stands as Roger Federer, Rafael Nadal, Simona Halep and Bianca Andreescu, among other big names, skipped the tournament because of the COVID-19 pandemic.

This year, with a proof-of-vaccination requirement for fans, spectators are back in the stands at Arthur Ashe Stadium. So, too, are the big names. Monday's early singles play saw matches contested between Andy Murray and Stefanos Tsitsipas, Andrey Rublev and Ivo Karlović, Halep and Camila Giorgi, Coco Gauff and Magda Linette and more.

The late action Monday is highlighted by a showdown between Marie Bouzková and Naomi Osaka, which gets underway at 7:50 p.m. ET. On the men's side, No. 2 Daniil Medvedev takes on Richard Gasquet at 9 p.m. ET.

Monday's full order of play can be found on the U.S. Open website. ESPN is broadcasting marquee matches, and outer-court matches are streaming on the ESPN app and ESPN.com.

The U.S. Open runs through September 12, with the women's singles final Sept. 11 and the men's singles final Sept. 12.

Top Men's Results

No. 3 Stefanos Tsitsipas def. Andy Murray 2-6, 7-6 (7), 3-6, 6-3, 6-4

No. 5 Andrey Rublev def. Ivo Karlović 6-3, 7-6 (3), 6-3

No. 8 Casper Ruud def. Yūichi Sugita 6-3, 6-2, 6-2

No. 11 Diego Schwartzman def. Ričardas Berankis 7-5, 6-3, 6-3

Top Women's Results

No. 5 Elina Svitolina def. Rebecca Marino 6-2, 6-3

No. 8 Barbora Krejčíková def. Astra Sharma 6-0, 6-4

No. 9 Garbiñe Muguruza def. Donna Vekić 7-6 (4), 7-6 (5)

No. 12 Simona Halep def. Camila Giorgi 6-4, 7-6 (3)

Analysis

The highlight of Monday's opening U.S. Open singles action was no doubt Andy Murray's return to tennis at Arthur Ashe Stadium, taking on No. 3 Stefanos Tsitsipas.

In a hard-fought, five-set match, Tsitsipas ultimately proved victorious, but Murray forced him to come from behind to do so.

Tsitsipas is looking for redemption after his shocking first-round loss at Wimbledon. He made it to the semifinals of the Australian Open.

After two hip surgeries in 2018 and 2019, Murray's future in tennis looked to be in doubt. Then, he injured his groin at the Miami Open in March.

The former world No. 1 has battled his way back to the ATP Tour, and holding the No. 3-ranked player to a narrow victory in a four-hour and 49-minute match can hardly be considered a setback.

Tsitsipas, who had a whopping 19 aces in the first-round match against Murray, will face France's Adrian Mannarino in the second round.

Fifth-seeded Andrey Rublev hit 19 winners against Ivo Karlović and was able to neutralize the Croatian's deadly serve.

Also at Arthur Ashe Stadium, Simona Halep took on fellow Grand Slam champion Garbiñe Muguruza and held on to defeat her in two sets. Halep had first-round exits in both the 2017 and 2018 U.S. Opens and didn't play in 2020.

Because of a left calf injury, Halep missed the French Open and Wimbledon, the two Grand Slam tournaments she's won.

Sloane Stephens is on to the second round as well after a three-set victory over fellow American Madison Keys that took two hours, 10 minutes. The two faced off in the 2017 final as well.

Stephens had two aces in the match, including this beauty:

The men's and women's singles first-round action continues Tuesday, with the men's and women's second round and the doubles first round beginning Wednesday.