LONDON—On Thursday, Jana Cepelova pulled off the biggest upset of Wimbledon to date by knocking out Roland Garros champion Garbine Muguruza in the second round.

A backhand into the net from the No. 2 seed sealed the 6-3, 6-2 win for 124th-ranked Slovakian.

Life at the top has been far from breezy for Muguruza ever since she stunned Serena Williams in the French Open final for her first career major.

“It was going to be a rough tournament for me after winning a Grand Slam and coming here,” Muguruza said. “Everybody, you know, like, looking at me.”

The 22-year-old fell in her first match on grass, right after Paris, in straight sets to Kirsten Flipkens in Mallorca. Arriving at Wimbledon in her new role as Grand Slam champion, she struggled in her opening round against tricky opponent Camila Giorgi, pulling out a 6-2, 5-7, 6-4 victory.

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“I’ve been through a lot of stages, and it's only half of the year,” Muguruza said. “I'm just dealing with it, honestly. I think it's hard. I'm doing [things] my way, trying to prepare every match, trying to be focused every practice. One day it's going to go my way, another day no. Hopefully my consistency will improve and I'll be able to be deep in all the tournaments.”

Muguruza was in real danger right off the bat on Thursday, and admitted that her energy level wasn’t where it should have been.

“I think my energy was missing a little bit today,” Muguruza said. “From yesterday I felt already a little bit, you know, tired … It's a tough day today.  I feel empty a little bit, and I start to be sick … I think she played great, with no fear.”

Cepelova’s biggest strength is her variety, and it paid off on the No. 1 Court.

“I think I played well,” Cepelova said. “She played sometimes [a] lot of mistakes … I like to play on grass. My style is, I think, that I change a lot [of] the shots. Sometimes I play flat, sometimes I play slices.”

Despite her low ranking, Cepelova has been a dangerous dark horse for the past few years, especially at the bigger events. In 2014, Cepelova reached her career-high ranking of No. 50, but last year she was set back by pneumonia and a back injury that helped push her ranking outside the Top 100.

The Slovak had beaten Top 10 players before Thursday, most recently defeating Simona Halep in the first round of Wimbledon last year and shocking Serena Williams in Charleston in 2014.

“I like to play against big stars, against the champions,” she said. “I try to keep winning against these champion players.”

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If anyone knows how Muguruza is feeling right now, it’s Angelique Kerber. The German suffered a similar fate after winning her first major at the Australian Open, losing in the first round of Roland Garros to Kiki Bertens. So far, the world No. 4 is safely through to the third round of Wimbledon with two straight-sets wins.

While Muguruza’s latest great run was a French Open title, Cepelova’s last deep showing was a semifinal appearance at a $50,000 International Tennis Federation Pro Circuit event in June.

If the 23-year-old had any advantage over the world No. 2, it was the grass surface. Cepelova came through qualifying, beating all three of her opponents in straight sets, before taking out Maria Duque-Marino—also in straight sets—in the first round.

Now Cepelova has an opportunity to take advantage of Muguruza’s draw, though it’s still not an easy road. She faces No. 28 seed Lucie Safarova next.

“Of course I would like to keep winning,” Cepelova said. “I have tomorrow [a] match against Safarova. I didn’t expect so much. She’s [a] lefty player. We are 1-1 [in head to head].”

The upset spells good news for Williams, who’s most imposing challenger in the draw was arguably the Spaniard.

Last year, Muguruza reached the final as the No. 20 seed before falling to Williams in straight sets. Now she’s getting used to life at the top, going up against players with nothing to lose on a regular basis.

“I think it's part of being at the top level,” she said. “I would not change that. I know every time I step on the court, well, they want to beat me so much.  They are kind of loose because they don't have nothing to lose.”

Life at the top is never easy, and the 22-year-old is learning that the hard way.

“I'm going to learn [from this] that you really need to concentrate on how to recover, and don't reach a moment where your energy is too low, especially to play a Grand Slam,” she said. “…You got to be ready. You cannot go out there not at your best.”