Andrea Hlavackova's hip ached every time she tried to move side to side - and tennis players spend most of their matches racing from one edge of the court to the other.
"You run to the right and you have a really tough problem to get back," she said of how she felt Saturday. "So I have to just go forward."
This late bloomer in the singles game is going forward in the U.S. Open and in her career. The 82nd-ranked Czech upset Maria Kirilenko 5-7, 6-4, 6-4 to reach the fourth round in her first individual appearance at the Open.
Now she might become known as more than just the player who comes from the family of brewers of the famous Pilsner Urquell beer in her home city of Plzen.
She's a 2011 French Open doubles champion, but don't call Hlavackova a doubles specialist. She switched coaches after Roland Garros this year and committed to maximizing her singles career - even though at age 26, she's considered old in tennis terms to make such a run up the rankings.
"I want to be in the big matches," Hlavackova said. "I want to be the winner of those big matches."
Hlavackova had never advanced past the second round at a Grand Slam event before this week or defeated a top-20 opponent in a completed match before beating the 14th-seeded Russian on Saturday.
In August, Hlavackova reached a career-best singles ranking. That earned her an automatic bid into the U.S. Open, where she had lost in singles qualifying five times.
Hlavackova collapsed to the court after Kirilenko's last shot landed wide. Tears streamed down her face during her post-match interview.
"This is the reason I play tennis," she said later.
Matches like her challenge in the fourth round: 14-time Grand Slam champion Serena Williams, likely in Arthur Ashe Stadium.
Hlavackova has never faced Williams in singles, but she knows how it feels to stand across the net from her. Hlavackova and Lucie Hradecka lost to the Williams sisters in the Olympic doubles final to earn a silver medal in London.
Asked if a doubles matchup is an accurate preview of Williams' intimidating presence, Hlavackova laughed and said, "Yes, it is."
Hlavackova marveled at the near impossibility of hitting a passing shot against either Williams: "Lob - no way. The side - they're there."
Hlavackova hopes to be healthy and rested for their match. She's scheduled for women's doubles and mixed doubles Sunday but might drop out of mixed.
Compensating for problems with her left hamstring, Hlavackova tweaked her right hip flexor warming up Saturday morning. So she went to the net 24 times against Kirilenko, winning 18 of those points.
Hlavackova insists her family's expertise meshes just fine with her career as a professional tennis player. Drinking one small beer a week is good for athletes, she explained.
Despite the storied history of Czech tennis, Hlavackova said confidence in her tennis game never came naturally.
"Americans, they have all this: `You're going to become No. 1. You're the best; you're the best.' They keep telling you all your life," Hlavackova said. "In Czech, you always have to prove yourself. You have to prove yourself to everyone - people, parents, opponents. I never actually had that attitude: I believe in myself."
Now she's starting to believe.