Update, 11:30 p.m. ET
Serena defeated Venus in three back-and-forth sets Tuesday night, advancing to the semifinals 6-2, 1-6, 6-3. She'll face Italy's Roberta Vinci, currently ranked 43rd in the world, who beat Frances Kristina Mladenovic 6-3, 5-7, 6-4.
With a win against Vinci, Williams would play for the U.S. Open title, and a Grand Slam of the year's major tournaments, the first since Steffi Graf's in 1988, The Associated Press reports.
But Serena got all she could handle from her 23rd-ranked sister, according to AP:
"She's the toughest player I've ever played in my life and the best person I know," Serena said in an on-court interview. "It's going against your best friend and at the same time going against the greatest competitor, for me, in women's tennis."
Updated 9 p.m. ET
Since the U.S. Open draw was released, fans and analysts alike immediately zeroed in on a potential quarterfinals matchup between World No. 1 Serena Williams and her big sister, Venus. Tonight, that match is here.
Serena Williams, 33, is three wins away from recording the first calendar Grand Slam in 27 years and cementing her place as the greatest women's tennis player of all time. But the first of those three tests comes tonight against Venus, in the sisters' 27th career meeting. Though Serena is favored — she has lost only two matches all year, and leads the head-to-head record between the two 15-11 — she's not underestimating her sister.
"I'm playing, for me, the best player in the tournament, and that's never easy," Serena said Sunday after her fourth-round win. "She's beaten me so many times. She's a player that knows how to win, knows how to beat me and knows my weaknesses better than anyone."
For 35-year-old Venus, this could be one of her last shots at making a deep push in a Grand Slam tournament. She knows what's on the line — for them both.
"I don't think anyone wants to be a spoiler," the older Williams told reporters Sunday. "I think people love to see history being made. ... But at the same time, you're focused on winning your match even though the circumstances are really much different than you."
The younger Williams has already won the "Serena Slam" twice, holding all four major titles consecutively both in 2014-15 and 2002-03, but she has yet to win them all in one calendar year. The last player, male or female, to do so was Steffi Graf in 1988.