With her 32nd birthday coming on September 9, Andrea Petkovic started pondering how much longer her 14-year tennis career might last, then showed she has plenty of time remaining.

The 88th-ranked German upset two-time Wimbledon champion Petra Kvitova 6-4, 6-4 Thursday to reach the third round of the US Open, matching her deepest run at Flushing Meadows since a 2011 quarter-final.

“I’m really glad. It was down to my mental fortitude,” Petkovic said. “It feels like a big win, not because it’s Petra but because of the last few weeks and how I reacted to that.

“Every tough moment in matches, I view it as a challenge. I don’t fall into self-doubt as I have before.”

By ousting the Czech sixth seed, Petkovic booked a third-round date with Belgian 25th seed Elise Mertens.

Not bad for a player who says she’s in the “fall” of her career.

“You don’t know how much longer your career is going to last so I’m really proud of how I’ve dealt with that,” Petkovic said.

“I’ve really been able to put the results aside and just try to become better and better in each moment.”

Petkovic, whose best Slam showing was a 2014 French Open semi-final run, reached a career-high ninth in the rankings in 2011 but knee, ankle and back injuries the next year brought an emotional letdown until she decided to continue her career.

“I chose to keep on playing. That makes everything easier,” she said.

“Since then I’ve been able to do what I want to do and still play tennis.

“I’m proud of being part of this (US Open) with the best women athletes in the world. Sometimes I think we paint ourselves a little smaller.”

And despite deeper Slam runs in younger days, Petkovic says her best tennis might be what she shows now.

“I consider myself a better player than where I was 10 years ago, but tennis has evolved so much since then,” she said. “Now if you don’t show up with your best tennis, you’re out in the first round.”

Belief big for Petkovic

Petkovic improved to 6-5 against Kvitova, whom she beat in last year’s Australian Open.

“I’ve beaten her before in Grand Slams and big matches. That gave me the belief. That’s the most important part if you want to win,” Petkovic said.

“It’s kind of her own fault. I admire and respect her so much. You’re 100 per cent focused. You don’t let anything stop you. I accept every winner she hits. I stay with it every point.”

Petkovic showed her abdomen in her post-win celebration, making good a threat to mock her trainer if he kept staring at his abs in a mirror, and called out friends who cheered her to victory.

“The strange people in the corner, those are my friends. It makes it so special I could win in front of them,” she said, adding later, “I think they are already at the bar. I’m not sure I want to see them right now.”