NEW YORK, NY, USA - No.20 seed Karolina Muchova won the last four games of an entertaining clash with two-time US Open champion Venus Williams to win, 6-3, 7-5, to reach the second round.

Despite trailing 3-5 in the second set, Muchova handed Venus her first first round loss at the USTA Billie Jean King National Tennis Center after one hour and 48 minutes under the lights of Arthur Ashe Stadium.

"I didn't even know that, but I just tried to stay focused and keep my serve," she explained during the on-court interview. "I lost my serve early in the second set, but I knew I would have chances, so I kept trying and it worked, so I'm very happy."

The Czech, who took a set from former World No.1 Naomi Osaka at the Western & Southern Open last week, made her first major breakthrough at the 2018 US Open, where she shocked two-time Grand Slam champion Garbiñe Muguruza en route to the third round. Last year, she defeated Karolina Pliskova in a 13-11 final set to make her first Grand Slam quarterfinal at Wimbledon.

"From the start, I wanted to be more relaxed and get into my game, but it was difficult because there were very short rallies. It was a very tough match, so I'm happy I turned it around in the end."

Taking on Venus for the first time, the 24-year-old, who is currently working with David Kotyza - former coach of Pliskova and Petra Kvitova - opened up a 0-40 lead in the opening serve game to draw first blood and rallied from a 0-40 deficit in her own opening serve game to consolidate.

Venus, who played sister Serena for three thrilling sets at the Top Seed Open presented by Bluegrass Orthopaedics, was playing her 22nd US Open, having won it in back-to-back years in 2000 and 2001 and returned to the semifinals for the first time in nearly a decade back in 2017. Battling through multiple marathon games, she clawed back from 5-1 down in the opening set, but was still struggling to find her range with 17 unforced errors, netting a forehand to hand Muchova the first set.

Muchova's 11 winners to eight unforced errors made for a tidier stat sheet to start, and began the second set with another hard-fought hold of serve. Errors began to creep into her game soon after, however, as Venus turned around a 40-30 game to move ahead by a break.

"To be honest, I'm really happy with my game," Venus said after the match. "I don't think I played badly tonight. I just needed to start faster than what I did. I'm pretty happy with how I played. She just played a little better.

"I love this game. I'm good at what I do. It's easy to stay motivated and excited about doing something so amazing. Not many people get to do this. I'm really fortunate and blessed to be someone who has gotten a chance to do it."

With Venus serving to force a deciding set, Muchova came back to life with a scintillating passing shot to set up two break points, converting her second as she won a third straight game.

Returning for the match, she continued to pressure the Venus serve, yielding a double fault to put her over the finish line.

"I didn't feel my game at the start of the year," Muchova said. "I was struggling a bit, but now I'm starting to gain confidence and feel my shots again. I'm trying to work on that and keep improving, match by match."

In all, Muchova maintained a positive differential, hitting 27 winners to 22 unforced errors, while Williams ended the match with 33 errors to 18 winners. Muchova earned a whopping 14 break points throughout the encounter, converting four while serving eight aces and saving six of eight break points on her own serve.

Up next for the rising Czech star, who greeted friend and Pitch Perfect star Rebel Wilson via online chat after the match, is Russia's Anna Kalinskaya, who defeated Nina Stojanovic of Serbia earlier in the day.

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