Played with uncommon ferocity and passion from both combatants, Serena emerged triumphant in the gruelling two hour 25 minute battle to reach the final four at Flushing Meadows for the first time since her 2002 win.
"I can’t believe I won," Serena said courtside. "Wow."
With the victory Serena edged to a 9-8 advantage in her head-to-head sibling rivalry with Venus.
Serena put on an amazing display of athleticism, racing from corner to corner to retrieve rocket forehands from Venus, stretching, straining and even sliding into a full split along the baseline trying to run down a blast.
The thrilling triumph sent Serena to a semi-finals match against sixth-seeded Dinara Safina of Russia, a 6-2 6-3 winner over Italy’s Flavia Pennetta. Friday’s other women’s semi-final will have second seed Jelena Jankovic going against fifth-seeded Olympic champion Elena Dementieva.
One of the semi-finalists will supplant Ana Ivanovic of Serbia as world number one at the end of the tournament.
Briton Andy Murray roared in relief as he reached his first grand slam semi-final after ending the U.S. Open campaign of Argentine teen-ager Juan Martin Del Potro 7-6 7-6 4-6 7-5.
The 21-year-old Murray meets the winner of Wednesday’s last match between world number one Rafael Nadal of Spain and unseeded American Mardy Fish.
It was going to be hard to top the tension of the Williams sisters clash, which featured brilliant rallies, raw power and such unbridled effort that the Arthur Ashe Stadium crowd gave them a standing ovation after one breathtaking exchange.
Venus ripped 36 winners but was guilty of 45 unforced errors. Her greatest lapse was while serving for the second set at 5-3, 40-0. She squandered three set points, making five successive errors that brought the set back on serve.
"I’m a very good closer," said Venus, a seven-times grand slam winner and like Serena a double Open champion. "I never had a match like that in my life. But I guess there’s always a first. I guess she played a little better.
"I think she gets back a lot more balls than the others. If it was someone else I probably would have won the match."
Venus led 6-3 in the second tiebreaker but three errors brought it back even. A backhand volley by Serena spoiled another Venus set point at 7-6 before two more errors by Venus ended it 9-7 in Serena’s favour.
"She played some great volleys and got a lot of balls back," said Venus, who beat Serena in the Wimbledon final in July for her fifth All England Club title.
"It’s not what I planned."
Murray plotted his path to the semi-finals through two dominant tiebreakers of his own, winning the first 7-2 and the second by 7-1.
The Scot overcame shaky moments in an error-filled third set before ending the slugfest as Del Potro served to stay alive.
"I’m very relieved," Murray said after the grudge match against the in-form, 19-year-old Argentine, with whom he traded on court insults at the Rome Masters in May.
Murray snapped Del Potro’s 23-match winning streak that included wins in his last four tournaments — on clay at Stuttgart and Kitzbuhel, and on hard courts at Los Angeles and Washington that sent his ranking soaring from 65 to number 17.
Del Potro, who was bandaged above and below his left knee and was limping toward the end of the quarter-final, said: "I just did my best. He played an unbelievable match. He’s a great player and that’s it."
Safina also registered a personal first at Flushing Meadows, reaching the U.S. Open semi-finals for the first time.
The 22-year-old Safina, runner-up at the French Open and the Beijing Olympics, raced through the first set and came from a break down in the second to clinch a place in the last four.
"It’s great," Safina said. "I’m getting closer to reaching the same thing as my brother (Marat Safin, the 2000 champion), so I hope that one day we can have the same titles."