World number one Serena Williams went within two wins of completing a calendar grand slam after beating sister Venus in three sets to reach the US Open semi-final.
The pair's 27th career meeting saw Williams junior prevail for a 17th time as she won 6-2 1-6 6-3 to set up a last four clash with Roberta Vinci.
"It's probably the toughest match I have played in a really, really, really long time where I wasn't actually beating myself," said Williams, who can become the first player since Steffi Graf in 1998 to win all four majors in a single year at with victory at Flushing Meadows.
"I was out there facing an incredibly tough opponent.
"It was gratifying because you're out there and you want to win so bad in that moment, and then when the moment is over -- because every single match I root for her every time, and so it's interesting to be in a position to what you're trying to win."
Elsewhere, men's number one Novak Djokovic powered into a semi-final meeting with defending champion Marin Cilic after seeing off Feliciano Lopez. The 2011 champion powered to the opening set but was pulled back by the Spaniard in the second, before completing a 6-1 3-6 6-3 7-6 victory.
"I thought I played a pretty good first set, and then after that, you know, I was just trying to hang in there and wait for the opportunities," the Serbian said. "I played good beginning of the third. Fourth was anybody's game, really. Didn't have many chances on his service games."
Elsewhere, despite the fourth round exit of Andy Murray and Johanna Konta in the men's and women's singles competition, British interest remains at Flushing Meadows after Jamie Murray and partner John Peers reached the last four of the doubles competition. The Wimbledon runners-up saw off fourth seeds Marcin Matkowski and Nenad Zimonjic in three sets.
"It was a close match, great atmosphere," said Murray, who is due to represent Great Britain in the Davis Cup against Australia. "It comes down to one or two points in the end and today it went for us so fingers crossed for the next one."