Karolina Muchova took advantage of an error-prone Venus Williams to advance 6-3 7-5 into the second round of the U.S. Open on Tuesday and hand the two-time champion her first opening-round loss at the tournament.
Muchova broke Williams to start the match and deployed a wide variety of shotmaking including a pinpoint serve, a slice backhand and some well-timed trips to the net to frustrate the former world number one.
Williams was sharper at the outset of the second set and served at 5-4 with the chance to extend the match to a third, but a backhand sailed on break point and Muchova leveled at 5-5.
The 24-year-old Czech held at love the next game and sealed the win when Williams issued her third double fault on match point.
"From the start I tried to be relaxed and get to my game but it was tough to get into a rhythm because we were having very short rallies," said Muchova, who hit 27 winners and saved six of her eight break points.
"I was happy I was able to turn it on in the end."
Williams was 21-0 in opening-round matches at the U.S. Open coming into Tuesday but the seven-times Grand Slam champion was undone by 34 unforced errors at Arthur Ashe Stadium, where her sister Serena looked on from the largely empty stands.
Despite the disappointment, the 40-year-old Williams, who turned professional at 14, said she was still passionate about the sport.
"I love this game," she said.
"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."
Next up for the 20th-seeded Muchova is Russian Anna Kalinskaya, who defeated Nina Stojanovic of Serbia in straight sets earlier in the day.