Evergreen Venus Williams turned back the clock at the Australian Open to book her first grand slam quarter-final in five years, while sister Serena earlier avenged last year's French Open loss to Garbine Muguruza.
Venus now faces an intriguing match-up with Madison Keys, the teenager touted as the future of American tennis.
Venus' 6-3 2-6 6-1 upset of Agnieszka Radwanska under the Rod Laver Arena lights proved there was still life in her 34-year-old legs and ensured the United States would have a third woman in the last eight after younger sister Serena edged Spaniard Muguruza in the day session.
Not since 2003 have three American women made it to the quarter-finals at Melbourne Park and 2004 was the last time at any grand slam.
Fittingly, the Williams sisters were the two U.S. women accompanying Meghann Shaughnessy into the last eight in Melbourne in 2003, with Serena, Jennifer Capriati and Lindsay Davenport making the quarters at the U.S. Open the following year.
"Now is my moment and I want to keep this moment going all year and then next year too. But that will take work"
Keys was four when a 19-year-old Venus captured the first of her seven grand slam titles at the 1999 U.S. Open. Now 19, she can reach her maiden semi-final in the majors with a win over Venus.
"It feels really good. My mom texted me before the tournament," Keys told reporters after defeating her namesake and compatriot Madison Brengle earlier in the day session.
"She said, 'it's your last grand slam as a teenager', and sent me a bunch of grandma faces.
"I'm like, 'thanks for reminding me, mom. Thank you. Love you. It's huge'. But it's my last slam as a teenager. I'm doing so well and hopefully I can keep it up."
Venus made her last quarter-finals appearance at a grand slam on her run to the 2010 U.S. Open semi-finals.
The following year she was diagnosed with Sjogren's, an auto-immune disease that can cause fatigue and joints pain, and has made her match-days a lottery.
"Things take time," Venus told reporters.
"Of course, I want to be playing deep in all my events. Everybody wants that. But it really just doesn't happen every time for everyone.
"Now is my moment and I want to keep this moment going all year and then next year too. But that will take work."
Serena Williams avenged last year's French Open loss to Garbine Muguruza, overhauling the hard-hitting Spaniard 2-6 6-3 6-2 to also reach the quarter-finals.
The 24th seed Muguruza stunned Williams in the second round at Roland Garros and another upset looked on the cards as the rising 21-year-old tore through the first set with a barrage of winners from the baseline.
Top seed Williams woke up and her serve, so often her saviour, became almost impenetrable.
After saving six break points in a marathon second game of the deciding set, Williams then broke Muguruza twice before sealing the match when her opponent blasted a forehand long.
Williams will play last year's losing finalist Dominika Cibulkova for a place in the semi-finals.
Feisty Slovakian Cibulkova scrapped like a street-fighter to grind down two-time champion Victoria Azarenka to reach the quarter-finals.
Cibulkova, beaten in last year's final by Li Na, weathered the inevitable comeback by the Belarusian blaster and ran her ragged in a match of the highest quality to earn the 6-2 3-6 6-3 win.
The pint-sized 25-year-old blasted 44 winners and broke Azarenka twice in the deciding set, sealing it when her opponent netted after two hours and 10 minutes.
