Coco Gauff of the U.S. during press conference after losing her fourth round match against Latvia's Jelena Ostapenko. Credit: Loren Elliott / REUTERS

(CNN) — Coco Gauff broke down in tears after her straight sets defeat in the fourth round of the Australian Open on Sunday, while fellow American Jessica Pegula progressed to the quarterfinals.

The 18-year-old Gauff lost 7-5 6-3 to Latvian Jelena Ostapenko on Margaret Court Arena and could not hide her disappointment when talking to reporters about the match afterwards.

Asked to explain her frustrations, a tearful Gauff said: “I think it’s because I worked really hard and I felt really good coming into the tournament and I still feel good. I still feel like I’ve improved a lot.

“But when you play a player like her and she plays really well, it’s like there’s nothing you can do.

“So it’s a little bit frustrating on that part.”

Gauff was the seventh seed in Melbourne and had started the season promisingly, winning the ASB Classic in Auckland earlier this month for the third WTA singes title of her career.

But Ostapenko held firm when it mattered, saving seven of the eight break points she faced, while taking all three of her chances on Gauff’s serve.

“I think every loss is somewhat in my control because I do feel like I’m a good player, but today she just played better,” Gauff added, per Reuters.

“There were moments in the match where I was getting frustrated because I normally can problem solve but today I feel like I didn’t have much answers to what she was doing.

“There were balls I was hitting deep and she was hitting them on the line and hitting them back deep, like, over and over again. It’s just one of those days that just didn’t go my way and went her way.”

Coco Gauff of the United States plays a forehand during the fourth round singles match against Jelena Ostapenko of Latvia during day seven of the 2023 Australian Open at Melbourne Park on January 22, 2023 in Melbourne, Australia. Credit: Mark Kolbe / Getty Images

Ostapenko is through to the quarterfinals for the first time in Melbourne Park and will play Elena Rybakina, who defeated women’s world No. 1 Iga Światek on Sunday.

Former French Open champion Rybakina, 23, defeated the 21-year-old 6-4 6-4 on Rod Laver Arena.

Światek was the tournament favorite after reaching the semifinals last year and winning eight titles in 2022, including two grand slams.

“I felt the pressure, and I felt that ‘I don’t want to lose’ instead of ‘I want to win’. So that’s, I think, a base of what I should focus on in (the) next couple of weeks,” Światek told reporters.

Rybakina, who was born in Russia but has represented Kazakhstan since 2018, won her maiden grand slam title at Wimbledon last year and the first for Kazakhstan.

Third seed Pegula is now the highest-ranked player left in the women’s draw after she defeated Czech Barbora Krejcikova 7-5 6-2 in one hour and 41 minutes.

The 2023 Australian Open runs through January 29 in Melbourne.