Fourth seed Serena Williams will face Dinara Safina in the US Open semi-final after she edged a tightly contested clash with sister Venus 7-6 (8/6) 7-6 (9/7) to book a place in the last four.
Serena trailed 5-3 in both sets and faced set points on both occasions but she battled back to secure a place in the semi-finals for the first time in six years.
The 26-year-old triumphed here in 1999 and 2002 and if she repeats that success this year she will become the world number one for the first time since a 57-week reign from 2002-03.
Serena has won three titles and reached the quarter-finals or better in 10 of 11 events played but remains without a Grand Slam title since the 2007 Australian Open.
Russian Safina had earlier defeated Flavia Pennetta 6-2 6-3.
Second seed Jelena Jankovic will meet Olympic champion Elena Dementieva in the other semi-final.