Jessica Pegula defeated her close friend Madison Keys to end her Australian Open title defence and settle a bet involving apple pie with cheddar cheese on top.

In an all-American encounter, Pegula needed just 78 minutes to complete an impressive 6-3, 6-4 victory and set up a quarter-final with compatriot Amanda Anisimova, who defeated China's Xinyu Wang later on Monday.

Defending champion Keys hit 26 winners - 12 more than Pegula -, but she was undone by 28 unforced errors on Rod Laver Arena.

Pegula not only walked away with bragging rights over her podcast co-host, but she also won a bet that means Keys must sample something, which is a Thanksgiving tradition for the Pegula family.

"In the US, we have Thanksgiving, and I don't know why, my grandparents are Canadian, but we have this tradition my mum always did," the sixth seed said in her on-court interview.

"Tell me if this is weird or if it's real. They would melt cheddar cheese on their apple pie."

Keys had previously insisted she would refuse to eat the delicacy, but she admitted defeat and said she would accept the terms of the wager.

"A bet is a bet, so I'll do it. I hope it's less gross than I think it's going to be, but we'll find out, I guess," Keys said in her post-match news conference.

Had the ninth seed won the fourth-round match and continued her title defence, Keys' side of the bet involved Pegula, whose billionaire parents own NFL side Buffalo Bills, wearing a Kansas City Chiefs jersey.

"She wanted me to wear a [Travis] Kelce slash Taylor Swift Chiefs jersey," Pegula said. "Honestly, I had a lot of motivation today not to wear that."

The 2024 US Open finalist wrote "no Chiefs jersey today" on a courtside camera lens after her victory, and the pair were seen chatting and laughing in the locker room straight after the match.

Keys, who beat Sabalenka in last year's final at Melbourne Park, said she was "still really proud of herself" despite her exit.

"Coming back and being defending champion, and dealing with all of the extra pressure and nerves - I am really proud of myself for the way I handled it," she said.

American quartet into quarters

Monday's remaining fourth-round results confirmed that the top six seeds in both the women's and men's singles have reached the last eight at a Grand Slam for the first time in the Open era.

Fourth seed Anisimova awaits Pegula in the quarter-finals after she overpowered Wang 7-6 (7-4) 6-4 to reach her first quarter-final at the Australian Open.

The 24-year-old has yet to drop a set at Melbourne Park as she bids for a first major title after being a beaten finalist at Wimbledon and the US Open last year.

With fellow Americans Coco Gauff and Iva Jovic already in the quarter-finals, it is the first time four players from the US have reached the last eight in the women's singles at the Australian Open since 2001.

Later on Monday, former Wimbledon winner Elena Rybakina stormed past 21st seed Elise Mertens to reach the quarter-finals for the first time since 2023, when she lost in the final to Aryna Sabalenka.

Kazakhstan's Rybakina raced to a 5-0 lead in the opening set before Mertens eventually got on the scoreboard, but the Belgian dropped serve at 3-2 in the second, and Rybakina saw out the match from there, wrapping up a 6-1, 6-3 win.

Rybakina, who hit 10 aces and 32 winners, will face second seed Iga Swiatek in the last eight.

Swiatek's career Grand Slam hopes still alive

Swiatek continued her quest to complete a career Grand Slam as she cruised past Australian qualifier Maddison Inglis to move one step closer to creating a piece of tennis history at Melbourne Park.

The Australian Open is the only major missing from the world number two's trophy cabinet.

Still focused on improving her semi-final run in last year's tournament, Swiatek took control of her fourth-round match against the last-standing qualifier Inglis on Rod Laver Arena.

But it was not complete control for the six-time Grand Slam champion, who struggled with her serve and unforced errors once again.

She only got 56% of her first serves in and made a total of 19 unforced errors across the two sets, which lasted one hour and 13 minutes, but could not stop hitting winners.

Despite still searching for her usual level, Swiatek executed a 31-minute bagel - a 6-0 triumph - in the first set against the world number 168.

But the Pole's chase for a seventh Grand Slam trophy never looked in doubt as she moved through the gears to secure a 6-0, 6-3 victory.

"I felt pretty confident from the beginning," Swiatek said. "I felt like the pace of the ball was different to the last round, so I needed to adjust my legs and be really precise with my footwork.

"I'm happy that I went for it today."