As Serena Williams sat stony-faced in the crowd, Vera Zvonareva played the match of her life to beat third seed Venus 2-6, 6-2, 6-4.
Defeat for the American rules out a fifth successive Grand Slam final between the siblings. Top seed and defending champion Serena had beaten Venus in the last four finals.
The French crowd rose as one and saluted the 18-year-old as she clinched the stunning win after an hour and 58 minutes of centre court drama.
"If you go to the court and believe you can win, you can win," Zvonareva said with tears in her eyes. "I did it."
Playing down the victory that gives hope to all who had begun considering the Williams sisters invincible, the Russian said: "I was just playing my game.
"I've played like this since I was six years old," she added, explaining the tireless running that wore Williams down.
"I'm just too tired to be excited," smiled the number 22 seed, before leaving courtside.
Zvonareva had made her Grand Slam debut here last year and on that occasion was beaten by Serena in the fourth round.
Sunday's victory was sweet revenge and, having said previously that her most memorable experience was losing to fellow Russian Nadia Petrova 6-0, 6-0 aged eight-years-old, the teenager may now want to think again.
Petrova also earned herself a memory to last a lifetime as she beat seventh seed Jennifer Capriati 6-3, 4-6, 6-3.
The two Russian youngsters will now meet in the quarters, guaranteeing a first Russian women's semi-finalist here since Natasha Zvereva, playing under the USSR flag in 1988.
While Petrova and Zvonareva will look back on their exploits fondly, Venus will want to erase Sunday's experience from her memory.
"Yeah, I was really off," she said. "It's very disappointing but at this junction there's nothing I can do about it now.
"I did what I could today, what else could I do?
"But I am definitely the same Venus... I couldn't change if I wanted to."
Capriati was similarly downbeat. "I thought I had pretty good chances in this tournament but it just goes to show you, you never know."
Venus's younger sister Serena earlier reached the last eight with a 7-5, 6-3 victory over Ai Sugiyama of Japan and will next meet Amelie Mauresmo, the French fifth seed who beat Spain's Magui Serna 6-1, 6-2.
The seasoned warrior marched straight into the quarter-finals with a performance of supreme confidence.
As his younger rivals have fallen by the wayside, the 33-year-old second seed has got stronger and tougher on the Roland Garros clay.
On Sunday, Flavio Saretta was his victim. The Brazilian had already ousted a former champion and world number one Yevgeny Kafelnikov. Agassi, though, is hewn from different stuff.
With a boldness borne from 18 years at the pinnacle of the sport, Agassi dominated centre court, sinking his opponent 6-2, 6-1, 7-5 in 103 minutes.
"I knew the first week was going to be crucial for me," he said.
"It turned out to be really crucial to get through that second round match (where Agassi had trailed Mario Ancic two sets to love).
"So now it's like basically a new life. I've sort of found my comfort zone out there."
Kim Clijsters will also be feeling a lot more comfortable after a remarkable comeback in her fourth round match.
The Belgian's relief was palpable when she recovered from the shock of losing the opening set of her match with Magdalena Maleeva in which she did not win a game.
In truth, Clijsters's 0-6, 6-2, 6-1 victory was largely down to her opponent's capitulation and the number two seed will have to be more on the ball when she faces Conchita Martinez in the last eight.
Martinez advanced when sixth seed Lindsay Davenport retired trailing 6-4, 2-0 with a right foot injury.
On centre court Agassi did what he has been doing for the best part of two decades -- he turned the screw, pushing the Brazilian ever wider and deeper until the youngster cracked.
It did not take long.
What was happening on the court became little more than a side issue. The match was won, and lost, in Saretta's head.
In the end, he was reduced to showboating, juggling a tennis ball around like a footballer. Brazilian to the end.
Unimpressed, Agassi finished him off soon afterwards.