Serena Williams remains on course to add to her title of 1999 and 2002
Fourth seed Serena Williams showed outstanding resilience to beat her sister Venus in straight sets and book her place in the US Open semi-finals.
Serena, 26, suffered an early break of serve but launched a gutsy fightback to take the first set on a tie-break.
Having already seen two set points go begging in the first set, 28-year-old Venus squandered a further eight in the second set tie-break.
Serena won 7-6 (8-6) 7-6 (9-7) and now faces Dinara Safina in the last four.
And Serena will be confident of reaching her 12th Grand Slam final after seeing off Venus with a show of grit and determination.
"I feel like I should have a trophy now," she said. "Unfortunately I don't and I've got to go to the next round.
"We played a great match today. It just boiled down to one point here and there. It could have gone anywhere. We were definitely playing the best (tennis of the tournament) so far.
"I feel like it was at least a semi or the final but it's not, so I'm hoping to play two more matches."
Safina, however, is unlikely to let 10 set points go begging as did Venus, who led 5-3 in both sets before collapsing.
The sisters were meeting for the 17th time in all competitions and it was Venus who made the more impressive start, dominating from the baseline and at the net to break in game three and take a 3-1 lead.
Serena was struggling with her movement and her decision to vary her game by combining baseline rallies with approaches to the net was backfiring.
But Venus buckled when serving for the set at 5-3 and 40-15, suddenly handing the momentum to Serena.
From nowhere, Serena began to play her most aggressive tennis of the tournament and always looked more relaxed during the tie-break.
A number of energy-sapping rallies left her looking tired by the middle of the second set, however, and Venus used her excellent approach play and venomous cross-court forehand to devastating affect.
Venus broke for 4-2 and was serving for the set at 5-3, but Serena dug deep once more and came back from 40-15 down to break back when Venus opened up and fired a forehand marginally wide.
In the second tie-break, Venus raced ahead but could not finish the job, blighting some fine tennis with a litany of unforced errors that saw four break points come and go.
On Serena's first match point, Venus fired long to hand Serena the win and an 9-8 lead in their career head-to-head record, 6-5 in Grand Slams.
Safina maintained her impressive form as she swept past Italy's Flavia Pennetta 6-2 6-3 to reach the semi-finals of for the first time.
The Russian, who has made the final in six of her last seven tournaments, broke the 16th seed at the first attempt and overpowered her in the first set.
Pennetta put up a better fight at the start of the second but lost her way and Safina sealed a comfortable win.
Safina, the sister of 2000 champion Marat Safin, had never previously been past the quarter-finals at Flushing Meadows, and the French Open runner-up said: "I'm getting closer to reaching the same thing as my brother."
Olympic champion and fifth seed Elena Dementieva faces second seed Jelena Jankovic in the other semi-final.