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.
The Williams sisters had played each other 26 times on the professional circuit, but their 27th contest was as highly anticipated as any. The early excitement was tempered by Serena’s excellent start, but Venus gave a reminder of why she is a seven-time major champion by taking it to a final set.
“She’s the toughest player I’ve ever played in my life and the best person,” said Serena.
“It’s going against your best friend and the greatest competitor, for me, in women’s tennis, so it was really difficult for me today.”
Venus said: “Losing isn’t fun, but probably the most gratifying part is I’m still very excited to see Serena have an opportunity to win the four majors.”