By Ravi Ubha, for CNN

(CNN) -- Garbine Muguruza lost her racket and her US Open match Thursday against a tricky foe who returned to the circuit this week after becoming a mom.

Tsvetana Pironkova upset the two-time grand slam winner at this unusual US Open 7-5 6-3 in her first event since leaving the game in 2017.

The Bulgarian -- once a Wimbledon semifinalist -- gave birth to a son, Alexander, before getting the urge to rejoin the circuit.

"Once you become a mother you don't magically lose your ability to play," Pironkova rightly pointed out to the WTA. "If you want to do it I don't think there's something that can stop you from doing it."

But she said becoming a mother changed her perspective.

"Your priorities have nothing to do with what they did before," she told reporters Thursday in a Zoom call. "Nothing is the same really. Right now I'm just happy to be playing. Having the opportunity for him to watch me also makes me really happy.

"It's just great to be playing without that extra pressure that I had on my myself before. Before it was like almost a life-and-death situation for me to win a match. Right now it's not really like that."

Pironkova said a rule change that came into effect last year also contributed to her decision to come back. Players who rejoin the tour after becoming a mom can use their last ranking to potentially get into a certain number of tournaments.

That ranking stood at No. 123 in the world, which would have normally forced the 33-year-old to go through qualifying.

But it was scrapped to limit the numbers on site as part of tennis' bubble amid the coronavirus pandemic and so Pironkova snuck into the 128-player singles draw in New York.

There is no junior event and mixed doubles, while women's and men's doubles fields have been reduced. Fans are being kept away, too.

Likes fast courts

There is, though, something new at the US Open: A supplier of the hard-court surface, which has generally had the effect of significantly speeding up the courts. The courts suit Pironkova, whose slice forehand in quicker conditions can cause real havoc to opponents.

It did at times to 10th seed Muguruza although the Spaniard -- who pulled out of last week's warmup Western & Southern Open citing an ankle injury -- led 5-3 in the opener on another hot, humid day.

After Pironkova rallied to take the first with a backhand winner, Muguruza flung her racket in dismay.

She then picked it up and smacked it against the court, which had the effect of breaking off the racket head and splitting the racket in two. Muguruza was left with the racket handle in her hand.

"I guess I'm more stronger now, I don't know," she said, smiling.

"Yeah, I'm not a type of player that does that a lot. I just feel like today out there was a tough first set. I felt I had it in control, then she played very well and took her chances. All of a sudden it was gone."

The Australian Open finalist saved five break points to level at 2-2 in the second set in what felt like it could be a turning point.

Muguruza, however, was broken at 3-4 and Pironkova cruised to victory, sealing proceedings with an ace. She not only served well but backed up her serve, winning 27 of 29 first-serve points, to hand Muguruza what is becoming a customary early defeat at Flushing Meadows.

Another early defeat for Muguruza in New York

While reaching the final at the three other majors, a quarterfinal in New York continues to elude her.

"The decision to come to the US Open I made two days before arriving here," said Muguruza. "It was really just giving me the chance to compete. I really didn't feel quite prepared."

Awaiting Pironkova in her first third-round appearance at a grand slam since the 2016 French Open is 18th seed Donna Vekic, who escaped in her opener before easing past another mom, Patricia Maria Tig, on Thursday.

Nine moms appeared in the draw at the start of the event, said the WTA, with the two of the highest profile, Serena Williams and Victoria Azarenka, still in contention.

Williams and Azarenka play in the night session, against Margarita Gasparyan and Aryna Sabalenka respectively.

Another grand slam winning mom, Kim Clijsters, fell in the first round to one of tennis' most in-form players prior to the pandemic, Ekaterina Alexandrova.