World number one Angelique Kerber despatched Japanese qualifier Risa Ozaki 6-2, 6-2 to set up a quarter-final clash with Venus Williams at the Miami Open.
Seven-time Grand Slam champion Williams fought past Russian seventh-seed Svetlana Kuznetsova 6-3, 7-6 (7/4) to reach the last eight.
Fourth-seed Dominika Cibulkova of Slovakia crashed out to the Czech Republic's Lucie Safarova and ailing French Open champion Garbine Muguruza also bowed out in the fourth round.
Kerber took charge early, breaking twice to establish a 4-1 lead and she was in firm control from then on.
The German said that while her opponent may only have been ranked 87th she had studied Ozaki's game closely before the contest.
"She came through the qualies, she beat good players, so I was ready for it,” Kerber said.
“I talked with my team and I knew that she's moving good and bringing a lot of balls back. I was trying to play my game and be aggressive."
Williams cruised through the first set before Kuznetsova, a two-time Grand Slam champion and runner-up at Indian Wells last week, fought back to take the second set to a tie-break.
It was a fierce battle for the decisive points, Kuznetsova going 4-1 up, but after two lengthy rallies, both of which were won by the 36-year-old Williams, the oldest player in the tournament emerged victorious.
The victory, sealed with a delicate slice at the net, was Williams' first against a top 10 opponent since 2015.
“It was tough out there," Williams said. "She is like a wall. To come back in the tie-break was great, I hope I can keep that tennis up."
Safarova, ranked 36th in the world, moved into the quarter-finals with a 7-6 (7/5), 6-1 victory over Cibulkova, which was her first triumph against a top 10 opponent since 2015.
Illness has disrupted Safarova's campaigns over the past two years but the seven-time WTA title winner, looks to be on the right track.
"I feel healthy now -- thank God and I'll keep trying to push my limits more and more,” she said.
The 30-year-old Czech will face Dane Caroline Wozniacki who advanced when Spain's Muguruza retired feeling ill after being edged in a first set tie-break.
In an all-Czech clash, number two seed Karolina Pliskova enjoyed a 6-1, 6-4 win over Barbora Strycova.
Pliskova will come up against Croatian Mirjana Lucic-Baroni whose impressive late career revival continued with the 35-year-old beating American Bethanie Mattek-Sands 7-5, 6-4.
- Halep hangs on -
Lucic-Baroni said she was benefitting from a more thoughtful approach to preparation.
"I've always worked hard, but we're working much smarter now," she said.
”We're not murdering me six hours a day anymore, but we're doing good things."
While she has never appeared in the Miami quarters before, this will be her third quarter-final appearance in her last four WTA events and she goes into the match knowing she beat Piliskova in the last eight in the Australian Open.
Romanian third seed Simona Halep showed her trademark fighting spirit in an enthralling win against Australia's Samantha Stosur, coming back from a set and 5-2 down in the second, saving a match point before going on to win the third set.
"I've been always like this," Halep said. "I never give up, I don't like to give up matches. I am here to play, away from home, I am going to give my all, always."