Red-hot Simona Halep also advanced to the quarterfinals with a 6-3, 7-5 win over 15th-seeded Flavia Pennetta. The third-seeded Halep has won her last 13 matches on tour, which includes her title at Indian Wells two weeks ago.
The seven-time Miami queen Serena eased past 24th-seeded former top-10 star and 2006 Miami titlist Svetlana Kuznetsova 6-2, 6-3 on Day 7.
Serena is 8-2 lifetime against the two-time major champion Kuznetsova, including 2-0 in Miami.
The 33-year-old nine-time Miami finalist Serena is the reigning Australian and U.S. Open champ. She beat Li Na in last year's Miami Open finale for a second straight title here.
Serena's quarterfinal opponent will be 27th-seeded German slugger and former Wimbledon runner-up Sabine Lisicki, who whipped 11th-seeded former French Open finalist Sara Errani 6-1, 6-2. Lisicki stunned Serena in a fourth-rounder at Wimbledon two years ago.
Meanwhile, the 16th-seeded three-time Miami winner Venus upset fellow former world No. 1 and No. 4 seed Caroline Wozniacki 6-3, 7-6 (7-1) on the hardcourts at Crandon Park.
"Caroline plays so well. Our last match went down to the wire, and this one did too," Venus said. "I just went for it, to be honest. I respect her so much that I knew I had to bring my best game."
The 34-year-old Venus is a perfect 7-0 lifetime against the Danish Wozniacki, who has taken only one set off the American in their seven career matchups. The American star also topped Wozniacki in Miami in 2008.
The seven-time Grand Slam singles titlist Venus titled in South Florida in 1998, 1999 and 2001 and was the 2010 runner-up.
Her quarterfinal opponent will be nifty 12th-seeded Spaniard Carla Suarez Navarro, who upended No. 7 seed and 2012 Miami champion Agnieszka Radwanska 5-7, 6-0, 6-4.
Florida native Sloane Stephens defeated Belinda Bencic, 6-4, 7-5 (7-5), ninth- seeded German Andrea Petkovic took out eighth-seeded Russian left-hander Ekaterina Makarova 6-1, 7-5, and 14th-seeded big-serving Czech Karolina Pliskova eased past Russian Daria Gavrilova 6-3, 6-2. Petkovic and Pliskova will square off in the quarters.