Serena Williams admits her sister Venus is the one player she fears at the US Open.
The most famous siblings in tennis will meet for the 27th time in the quarter-finals at Flushing Meadows after Serena ousted fellow American Madison Keys 6-3 6-3 and Venus thrashed Estonian qualifier Anett Kontaveit 6-2 6-1.
It is the first all-Williams clash in New York since they met at the same stage in 2008, when Serena edged two tie-breaks en route to winning the title, and their 14th in grand slams overall.
Serena has enjoyed the ascendancy in recent years, winning eight of their last 10 major battles, including most recently in straight sets at this year's Wimbledon.
This latest family affair, however, carries extra poignancy given Venus could prevent her younger sibling from becoming the first player since Steffi Graf in 1988 to complete the calendar Grand Slam.
Venus' demolition of Kontaveit continued the 35-year-old's superb form in New York, which also saw her knock out rising star Belinda Bencic of Switzerland in round three.
"For me I'm playing the best player in the tournament, and that's never easy," Serena said.
"She's beaten me so many times. I've taken a lot of losses off of her - more than anybody.
"She's a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone.
"It's not very easy. Like I said, for me, she's the only player in the draw I don't want to play - not only because she's my sister but because for me she's the best player."
Serena would become only the fourth female player in history to win all four major titles in a single year and is chasing her seventh success at the USOpen.
"I don't think anyone wants to be a spoiler," Venus said.
"I think people love to see history being made.
"No one is out to be a spoiler, but at the same time, you're focused on winning your match even though the circumstances are really much different than you (would want)."
The sisters boast 28 major singles titles between them, 21 for Serena and seven for Venus, and have played each other in eight grand slam finals.
Their first competitive meeting came 27 years ago in the Australian Open second round when Serena won 7-6 (7/4) 6-1.
"I think it's more fun than it used to be," Serena said.
"We really relish the opportunity. We're both happy to still be involved in getting so far.
"And it's still super intense. She's doing well and she wants to win this. So do I. It's not easy.
"I think it's been an amazing rivalry. I think it's meant a lot. We've done a lot for the sport. Hopefully it can continue as long as we play."
Hoping to play the winner of the Williams quarter-final is Italy's Roberta Vinci, who progressed to the last eight after Eugenie Bouchard pulled out with concussion.
Bouchard, seeded 25th, injured her head after slipping and falling in the locker room following competition on Friday evening.
The Canadian dropped out of the doubles and mixed doubles but had delayed her decision regarding singles, before confirming her withdrawal on Sunday.