Serena Williams overcame her sister Venus at the US Open to move within two victories of her first calendar Grand Slam.
The world number one held her nerve to win 6-2 1-6 6-3 and reach the semi-finals in New York.
Serena, 33, will play unseeded Italian Roberta Vinci in the last four on Thursday.
The American could become the first player to win all four majors in the same year since Steffi Graf in 1988.
Venus, 35, was playing in the fourth round at Flushing Meadows for the first time since 2010, but threatened to derail her sister's grand ambitions.
The 23rd seed battled back to force a final set, only to see younger sister Serena capitalise on an early service break and claim her 33rd straight win at the Slams.
Top seed Novak Djokovic will play defending champion Marin Cilic in the US Open semi-finals on Friday.
Djokovic, 28, beat Spanish 18th seed Feliciano Lopez 6-1 3-6 6-3 7-6 (7-2) in a sweltering New York night session.
The Serb unleashed an angry tirade towards his player box after the second set but finally prevailed at 01:00 local time.
Croatia's Cilic withstood a Jo-Wilfried Tsonga fightback to win 6-4 6-4 3-6 6-7 (3-7) 6-4.
On a sweltering day at Flushing Meadows, Djokovic and Lopez were still suffering in 27C and 66% humidity as midnight came and went.
Djokovic had dominated the opening set but was then furious at dropping the second, thumping his racquet on his bag at the changeover.
The world number one regained control in a lengthy Lopez service game at the start of the third and took a tight fourth in a tie-break after two hours and 39 minutes.
"He's one of the rare players that actually serves volleys on the first and second serve," Djokovic said.
"I'm just glad to get through in four. Obviously, it was frustrating at times but this is quarter-finals of a Grand Slam, this is what you expect."