PARIS -- American qualifier Kayla Day advanced to the third round at a Grand Slam on Thursday for the first time in her career with a 6-2, 4-6, 6-4 win against No.20 Madison Keys at the French Open.  

Here are a few things to know for the 23-year-old California, who's had a tough journey to get to this point:

Her Czech heritage has helped her tennis in more ways than one

Day's mother is from the Czech Republic and instilled a deep love of tennis in her daughter. In fact, Day carries dual citizenship with the United States and the Czech Republic. 

"I have a Czech passport, which is very helpful when I come to Europe," Day said. "My mom, she was born and raised in Prague, and I speak fluent Czech.

French Open Day: Scores | Order of play | Draw 

"That's the only reason why I'm good at tennis, because I'm half Czech," Day quipped. 

From Martina Navratilova to Petra Kvitova, the Czech Republic has a storied history of producing talented left-handers. That fact isn't lost on the 23-year-old from Santa Barbara, California, though any hopes that she would be the next Kvitova was quickly dispelled early in her career.

"A really long time ago I told my coach at that time, 'Oh, I like Kvitova.' I think he asked the question, like, 'Who are you most similar to?' I said Kvitova.

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"He said, 'What are you talking about? You play nothing the same. Get that out of your mind.'

"Yeah, I wish, she's obviously a Grand Slam champion, an incredible player, but yeah, I'm not super similar to Kvitova in the way we play."

She has a Hall of Famer in her coaching box

Day, who in beating Keys knocked off a Top 20 player for the first time in her career, has been working with Pat Cash since the start of the season and credits him with improving her footwork on clay. She first met the 1987 Wimbledon champion during the 2021 offseason when he was working with China's Wang Qiang. 

"We hit together a few times," Day said. "From there, I kind of saw him around and we would be friendly with each other. Then this year in Australia he offered to help me a little bit before my other coach came. Ever since then, we've been working together and it's been great.

"He's just completely changed my idea on movement and the way I move. We did a lot of work before the clay season started together. So he's made probably the biggest difference in my movement, especially on my backhand side. Before I couldn't even really hit an open-stance backhand, so he helped me through that."

She had her first breakthrough in 2017

The last time Day made her splash at a big tournament was more than six years ago at the 2017 BNP Paribas Open. There, as a 17-year-old wild card ranked No.175, Day scored wins over Kurumi Nara and Mirjana Lucic-Baroni to make the third round.

"I grew up watching that tournament from when I was super young," she said. "My mom would take me down there and we'd watch. So to be able to play on that center court was just like a dream come true to me. It was like a full-circle moment to grow up watching that tournament and play there."

As luck would have it, Day faced Garbiñe Muguruza. At the time, the Days had two dogs. One was named Rafa, after the Spanish lefty who Day idolized. The other was named ... Garbiñe. 

"The two tennis player dogs are still around, but they're getting up there," Day said.

Injuries and illness derailed her development

Day reached a career-high No.122 in July 2017 but struggled to build on the momentum from her Indian Wells run. 

"I had a ton of injuries, and I also had mono, which made me feel not so well for a very long time," she said. "I tore my quad. I fractured my foot. I tore both labrums in my hip. So it was just a lot of bad luck, one thing after another.

"Every time I tried coming back, it just felt like something else would happen. So then when I started feeling good, it was in 2020 during COVID, and they canceled all the tournaments and my ranking had dropped to like 600 at that point. So it was really tough to even try to get into a tournament to try to play."

After working her way back into the Top 200, Day found traction on the clay. Since the clay season began in Charleston, she is 19-5 across all levels, including a title run at an ITF W100 in Bonita Springs. After losing out to Emma Navarro in the USTA Roland Garros Wild Card Race, Day earned her French Open main-draw debut via qualifying. 

"Yeah, I found my way back," she said with a smile.

She was a junior US Open champion

At 16 years old Day stormed her way to the US Open girl's title in 2016. The only player to take a set off her? Bianca Andreescu. The Canadian ultimately bowed out, losing 5-7, 6-1, 6-2 in the semifinals. Three years later, Andreescu was a US Open champion. 

Still chasing her Top 100 debut, Day's run in Paris will inch her ever closer to rubbing shoulders with her junior contemporaries more regularly on the WTA Tour. 

"I've definitely learned that I'm a lot tougher than I thought I was, to just grind my way back like I have," Day said. "That's something I'll carry with me through the rest of my career. Because a lot of people, I think, counted me out or didn't believe in me anymore or whatever, and I just worked super hard and trusted myself.

"And, yeah, now I'm back here."