Facing unfamiliar territory in more ways than one on Monday night at Roland Garros, Iga Swiatek passed the test posed by both the conditions and her Ukrainian foe Marta Kostyuk with flying colors.

Playing her first scheduled night session match on Court Philippe-Chatrier against an opponent 13 months younger than her, the defending champion and No.8 seed sealed the eighth and final berth in the quarterfinals with a hard-fought 6-3, 6-4 victory. 

Not often the elder of the two when she takes the court, the 20-year-old had her hands full with an 18-year-old Kostyuk, who was playing in the fourth round of a Grand Slam for the first time.

A break down early in the first set and pushed all the way in the second, Swiatek needed to summon all her experience on the main stage in Paris to secure a 92-minute win.

"Playing the night session... is always hard, because you have to change the whole rhythm of the day, change the routines," Swiatek said after the match.

"So still, even though I played many night sessions right now, I sometimes struggle at the beginning to find the rhythm and to see the ball properly with the lights, not with the sun.

"You just have to adjust to that, because we both have the same conditions. It's going better and better every match, but still, I need some more experience.

"I like [being tested] because it keeps me constantly on my toes. It just keeps you focused all the time, because you don't have that flow, so... you have to work all the time.

"With that flow, you sometimes can seem surprised that, 'Hey, something is not good,' and then you're going to panic. But right now I'm getting tested, as you said. Still I'm moving forward, so that's even better for me."

Six of the nine games played in the first set stretched past deuce, including a six-deuce fourth game where Swiatek earned the break back after dropping her own serve at 1-1.

The pair later swapped service games at the same stage of the second set before Swiatek earned a second at 3-3, the break that eventually decided the match. She denied Kostyuk a chance to pull level at 4-4, and though the Ukrainian saved a match point in the eighth game to force Swiatek to serve for the match, the No.8 seed would ultimately not be denied on her second chance. 

"It was really intense. It was just one of the points when I couldn't finish her and she was fighting till the end," Swiatek said of her thoughts on match point.

"I'm pretty happy that my lob was in, because I couldn't actually see. But it was risky, so I felt like kind of shaky, but I'm really happy that I made it in [an] important point. I was pretty solid so I'm happy about that."

The win gives Swiatek a pair of 10-match winning streaks, both overall this season and at Roland-Garros as her sets-won streak in Paris extends to 22. She's also one of four quarterfinalists to still be alive in the singles and doubles fields, joining Barbora Krejcikova, Elena Rybakina and Anastasia Pavlyuchenkova. 

Up next for Swiatek is a first-ever meeting against No.17 seed Maria Sakkari, who ousted last year's runner-up, No.4 seed Sofia Kenin, in straights earlier in the day.

"I haven't played her, so it's hard to say. I played many practices with her, and she's really cool girl. Our teams, we like each other," Swiatek said.

"It's never easy to play with one of the nicest girls on tour. Tactically, I'm going to for sure be prepared. Right now I'm not thinking about it, because still I have tomorrow doubles. So I just want to focus on getting my body back to being fresh.

"Hopefully, it's going to be good. She's the kind of player who likes to play defense as well, as my opponent today. I'm just going to be ready. I'm going to be patient and hopefully it's going to get results."