US Open 2021: Stefanos Tsitsipas posts controversial win over Andy Murray, Naomi Osaka and Caco Gauff advance

It's the first day of an action-packed first round at the US Open. With 64 matches happening today, there will be plenty of upsets and tight matches, and all will be seen by fans attending in person for the first time since 2019.

Check back here for the latest updates on all the happenings at the 2021 US Open.

Andy Murray 'lost respect' for Stefanos Tsitsipas in 5-set loss

Andy Murray threatened a big first-round upset on Monday, opening a 2-1 lead over No. 3 seed Stefanos Tsitsipas. But Tsitsipas rallied to win the final two sets in a marathon match that lasted nearly five hours to advance to the second round with a 2-6, 7-6, 3-6, 6-3, 6-4 victory.

The final set started amid minor controversy with Murray complaining to chair umpire Gerry Armstrong about Tsitsipas' extended break between the fourth and fifth sets. Murray apparently wasn't thrilled to grant Tsitsipas some extra bathroom time amid the 4:49 match. 

"I lost respect for him," Murray said of Tsitsipas after the match. 

Murray dropped the first two games of the final set and lost a chance to break Tsitsipas in the sixth game after winning the first two points. The 34-year-old 2012 US Open champion had the crowd behind him, but couldn't rally after Tsitsipas extended his fifth-set lead to 4-2.

Tsitsipas advances to face France's Adrian Mannarino, who also needed five sets to secure his first-round matchup over countryman Pierre Hugues Herbert.

Naomi Osaka cruises through first round

In her first Grand Slam match since that fateful opener at the French Open, Naomi Osaka looked ... well, like Naomi Osaka.

The two-time US Open champion advanced with a 6-4, 6-1 win over Marie Bouzkova of the Czech Republic. The first set was competitive throughout, but Osaka was simply gaining steam. She won seven straight games between the end of the first set and most of the second set.

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20-year-old Alycia Parks ties Venus Williams record

Let us present a story in two tweets.

One sees Venus Williams now longer needing a wildcard to qualify for the US Open, which allowed 20-year-old American Alycia Parks to make the tournament at the absolute end of the field.

In the other, Parks hits a 129 mph serve to tie Williams' US Open record.

Parks would officially lose 6-3, 7-5, but she can at least she made an interesting bit of history in New York.

Coco Gauff rallies after losing first set

Coco Gauff dropped her first set on Monday to Poland's Magda Linette at Louis Armstrong Stadium. But the 17-year-old American bounced back in the second and fended off a rally form Linette in the third to secure a 5-7, 6-3, 6-4 victory to advance to the second round. 

Gauff broke Linette twice to start the third set en route to a 4-0 lead. But Linette ensured she wouldn't be swept in the decisive set, breaking Gauff while holding serve twice to cut Gauff's lead to 4-3. But Gauff wouldn't be broken again, and won on her final serve to set up a second-round matchup with fellow American Sloane Stephens.

Sloane Stephens defeats Madison Keys

As she did once before, Sloane Stephens has beaten Madison Keys at the US Open. 

In a repeat of the 2017 US Open finals match that gave Stephens her first Grand Slam title, she defeated Keys 6-3, 1-6, 7(9)-6 in a USA vs. USA Round 1 match. 

Stephens battled some nerves at the start of the match, letting Keys take the first game and get a lead in the second. She found her feet quickly, and though Keys won two more games, Stephens roared back to take the first set relatively easily.

But things turned around entirely in the second set. Keys absolutely dominated, winning 6-1. The third set was closely contested, with constant reversals of fortune. Keys and Stephens were so evenly matched that in the third set, neither of them ever led by more than one game. Knotted at 6 each, they went to a tiebreaker that had Keys saving match point two different times. But Stephens eventually closed it out to advance to Round 2.

John Isner upset in first round

The top-seeded American man at the US Open did not last long.

John Isner, the tournament's No. 19 seed, fell in straight sets to countryman Brandon Nakashima 7-6 (9), 7-6 (8), 6-3. It's not often you see Isner lose the battle of dominant serves, but the 20-year-old Nakashima won 80 percent of his service points to post his second ever US Open win.