Serena Williams labelled sister Venus as the "best player" at the US Open ahead of the duo's quarter-final at Flushing Meadows.

The Williams sisters won through on Sunday to set up a 27th meeting, but first at the year's final grand slam since 2008.

Respectful of her older sister, Grand Slam-chasing Serena said seven-time major winner Venus was the best player in New York.

Numbers and recent form would suggest otherwise, with Serena chasing a 22nd grand slam title and holding a 15-11 win-loss record over her sister.

"Well, I mean, I'm playing, for me, 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. Yeah, she's a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone. So it's not an easy match at all.

"Hopefully things will go right."

Serena proved far too good for fellow American Madison Keys on Sunday, winning 6-3 6-3 in one hour, eight minutes.

If any player was to end her bid to become the first player since Steffi Graf in 1988 to win the Grand Slam, Serena said Venus was ideal.

"I would rather lose to Venus as opposed to anyone else," she said.

"I, in general, don't like to lose. So hopefully we'll just go out and do the best that we can, and whatever happens hopefully have a good attitude about it."

On Venus' game, Serena said: "Well, we play a lot. She plays similar. She's fast; I'm fast. She hits hard; I hit hard. She serves big; I serve big. We have a very similar game.

"We've had the same coach for a long time. It's like playing a mirror. I have to be really ready."