Serena Williams fought off 10 set points to prevail in the greatest duel yet with sister Venus, a heart-stopping 7-6 7-6 victory that put her in the US Open semi-finals on Wednesday.
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.
The eight-times grand slam winner turned away two set points in the first 8-6 tiebreaker and eight more in the second set, including four in the final 9-7 tiebreak, which ended with a Venus forehand drive landing beyond the baseline.
"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.
"I think we played a great match today," Serena said. "It just boiled down to one point here and there."
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 win sent Serena to the semi-finals against sixth seed Dinara Safina, a 6-2 6-3 winner over Italy's Flavia Pennetta. Friday's other women's semi-final will have second seed Jelena Jankovic against Olympic champion Elena Dementieva.
One of the semi-finalists will supplant Ana Ivanovic as world number one at the end of the tournament.
Top seed Rafael Nadal, looking for his first U.S. Open title to underline his new status as world number one, booked a date with Briton Andy Murray in the men's semi-finals.
The Spaniard closed Wednesday's marathon night session with a 3-6 6-1 6-4 6-2 victory over unseeded American Mardy Fish.
Delayed by a series of long matches, Nadal clinched his first trip to a US semi-final at 2:10 am with a boisterous throng of fans still on hand in the city that never sleeps.
"Thanks everyone for being here 'til 2 o'clock," Nadal told the crowd in a courtside interview. "The night atmosphere here is always amazing. But I prefer to finish a little earlier."
Murray gave a roar of relief after ending the 23-match winning streak of Argentine teenager Juan Martin Del Potro 7-6 7-6 4-6 7-5 to reach his first grand slam semi-final.
The biggest buzz of the day came from the clash between the Williams sisters 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.
US Open champion in 2000 and 2001, Venus failed to capitalise on serving for the first set at 5-4 and again for the second at 5-3.
Venus ripped 36 winners but had 45 unforced errors in her quest for the quick kill. Her greatest lapse came when 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."
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 semis through two dominant tiebreakers, winning the first 7-2 and the second 7-1. The Scot overcame shaky moments in an error-filled third set before ending it as Del Potro served to stay alive.
"I'm very relieved," Murray said after the grudge match against the 19-year-old Argentine, with whom he traded on-court insults at the Rome Masters in May.
Del Potro had won his last four tournaments -- on clay at Stuttgart and Kitzbuhel, and on hard courts at Los Angeles and Washington to soar from 65 to 17 in the rankings.
Safina, 22, also registered a personal first, reaching the US Open semi-finals for the first time.
"It's great," said Safina, runner-up at the French Open and Beijing Olympics.
"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."