Rybakina was more efficient than Keys, winning more than 80 per cent of her first-serve points and converting three of her eight break opportunities during the 84-minute encounter.

"Really happy with my performance today," Kazakhstan's Rybakina said in her on-court interview.

"It was a tough one. With Madison it's always difficult matches and she's an aggressive player.

"I knew that I needed to be ready for every point, especially on the return."

Rybakina, who due to illness withdrew before her first match at the recent Indian Wells tournament - where she was defending champion - moves on to the eighth quarter-final in her last 10 WTA 100 events.

Another player from Kazakhstan, Yulia Putintseva, beat Ukraine's Anhelina Kalinina 6-4 7-6 (7-5).

Sakkari, seeded eight in Miami, was scheduled to play in Monday's opening match but advanced without hitting a ball after Russia's Anna Kalinskaya withdrew ahead of their fourth-round match due to health issues.

Kalinskaya, the 22nd seed, arrived in Miami following a third round defeat at Indian Wells where a stomach ailment forced her to miss a few days of practice.

The 25-year-old Kalinskaya reached the round of 16 in Miami without dropping a set in wins over China's Wang Xiyu and Latvian ninth seed Jelena Ostapenko.

"I'm so sorry I was really looking forward to playing today, but unfortunately my health doesn't allow me," Kalinskaya said.

Rybakina owns a 2-1 record in head-to-head meetings with Sakkari, who arrived in Miami fresh off her loss to world No.1 Iga Swiatek in the Indian Wells final.

"Tough opponent and I'm going to try to do my best to prepare as much as I can and try to be fresh physically," Rybakina said.

Australian Associated Press