Serena Williams of the United States reacts during her Women's Semi final match against Timea Bacsinszky of Switzerland on day twelve of the 2015 French Open at Roland Garros on June 4, 2015 in Paris, France

Serena Williams of the United States reacts during her Women’s Semi final match against Timea Bacsinszky of Switzerland on day twelve of the 2015 French Open at Roland Garros on June 4, 2015 in Paris, France

*“I don’t think I’ve ever been this sick,” Serena Williams said after winning her semifinal match today against Timea Bacsinszky 4-6 6-3 6-0 while battling the flu. “I didn’t expect to win that, and I can’t believe I won.”

Williams was noticeably lethargic through the first set and most of the second. At 2-2 in the second set, she held off five break points before eventually losing that game. It looked like Bacsinszky had control of the match. Serena was coughing non-stop and at one point during the changeover appeared on the verge of throwing up. The announcers wondered why she wasn’t calling for a trainer.

“I really thought I was going to lose, and I thought: if I’m going to lose, I might as well go for winners. I was so tired, and I just fought for every point,” Williams said in an on-court interview after the match. She said if she figured if she was going to lose, she wanted to at least go down with a few winners.

Whatever Williams coughed up during that short break, it changed everything. She came back out and broke Bacsinszky to even the set at 3-all.

She hit two aces to close out the next game, and suddenly the 19-time Grand Slam champion was back on top, leading 4-3. A dominant overhand slam sealed the eighth game and the 5-3 set lead. She’d suddenly come alive.

Her entire demeanor changed. She even started to look a bit energetic. Holding serve, she took the second set 6-3, and then the final set 6-0.

Serena Williams of the United States goes to the net after victory in her Women's Semi final match against Timea Bacsinszky of Switzerland on day twelve of the 2015 French Open at Roland Garros on June 4, 2015 in Paris, France

Serena Williams of the United States goes to the net after victory in her Women’s Semi final match against Timea Bacsinszky of Switzerland on day twelve of the 2015 French Open at Roland Garros on June 4, 2015 in Paris, France

“I got the flu after my third-round match and I haven’t been getting better,” Serena said after the match. “Hopefully this is the worst and I’ll get better.”

She’ll have a day to rest up before facing Lucie Safarova in the Women’s Finals.

Williams already claimed the Australian Open title earlier this year. If she can capture the French Open title this weekend, she’ll be the first woman to win the first two slams in the same year since Jennifer Capriati did so in 2001.

The last man or woman to complete the calendar-year Grand Slam was Steffi Graf in 1988.