NEW YORK, United States (AFP) — Defying an energy-sapping disease, Venus Williams has pushed herself farther than she ever imagined at age 37 to reach her 39th career Grand Slam quarter-final at the US Open.

US ninth seed Williams, seeking her eighth Grand Slam title and third US Open crown, defeated Spain’s 35th-ranked Carla Suarez Navarro 6-3, 3-6, 6-1 Sunday.

She booked a Tuesday fight for a semi-final spot against Czech 13th seed Petra Kvitova, a two-time Wimbledon winner who defeated Spanish third seed and reigning Wimbledon champion Garbine Muguruza 7-6 (7/3), 6-3.

Williams, this year’s Wimbledon and Australian Open runner-up and the oldest woman in this year’s US Open draw, is only 1-4 against Kvitova, but she has grown accustomed to overcoming the odds.

In 2011, Williams was diagnosed with Sjogren’s disease, which leaves her weakened and pushed her into a vegan diet to stay competitive and combat the illness.

“Over time I’ve learned that I can push myself a lot further than I thought I could,” Williams said. “I do have to push myself a lot. I think that’s the biggest thing that I’ve learned.

“In the beginning, when everything is easy, you’re healthy, it’s easy to play when you don’t have those odds against you. But when you have the odds against you, it’s hard to find that in yourself every day.”

Williams, 19-3 in majors this year, said she wasn’t sure how she was able to stay competitive over rivals sometimes half her age.

“I love the pressure and I need a challenge,” Williams said. “I don’t know (how I sustain it). If I could do it I would put it in a box and sell it.”

Williams said health, fitness, skill and love for the competition all play a role.

“You do have to have a love because it’s a lot of work,” she said. “There are sacrifices you make with your family, your loved ones — just doing normal things like sitting on your own couch that are taken for granted. You have to have the love to deal with the pressure as well.”

Playing for 20 years at the US Open and winning five Wimbledon crowns has made her a target the moment she steps on the court.

“It’s never easy to win a Slam. Now I’ll play players who I’ve never seen or know their name and you can’t let up one point,” Williams said.

“Players come out with a lot more confidence now. Every time I step out on the court, that person is coming for me. It doesn’t matter if I played for a year, not even if I’m ranked 100 or number whatever, they’re coming for me. When I get to these majors, I know that’s what it is.

“They’re playing well. They’re not going to give you any matches. In the case if they do, I will accept it as well if they don’t play as well as they want. I’m accepting all major credit cards and unforced errors.”

Williams could return to the world top five for the first time since January 2011 if she beats Kvitova and reach world number two if she takes the title, although she hasn’t played three Slam finals in a year since 2002.

Her win also kept alive host-nation hopes for an all-American US Open women’s semis, with Sloane Stephens, Jennifer Brady and Madison Keys also in contention.