Venus Williams of the US celebrates after defeating Anett Kontaveit of Estonia during their fourth round match at the US Open Championships tennis tournament in New York, September 6, 2015. [Photo/Agencies]

Venus, of course, knows full well the challenge that awaits Tuesday.

"What else can you do, except try to win the point and hope she doesn't hit an ace?" said Venus, who lost in the third round or earlier at each of the past four US Opens.

The sisters' mother, Oracene Price, said she wouldn't attend the quarterfinal.

As for whether Venus will be in a tough spot _ hoping to win, yet also well aware of what her sibling is pursuing _ Price said: "I know it's going to be hard, because I know (Venus) wants (Serena) to get it."

Serena acknowledged having a bout with the jitters entering her second-round match, when she wound up double-faulting 10 times and made another two dozen unforced errors against a qualifier ranked 110th. Afterward, she took pointers from Mouratoglou and went to the practice court right away.

In the third round, against someone ranked 101st, Serena dropped the first set and was two games from defeat in the second before turning things around. Again, she put in more work to fix things.

"I was like, `Serena, it's now or never. You've got to get that serve together,"' she said after winning 22 of 28 first-serve points and never facing a break point against Keys.

And what about heading back out for a training session with Mouratoglou?

"No, not today," Serena said. "I'm going to take the rest of the day off and relax and just enjoy it."

Keys played well, too, at the outset, and appeared relaxed, managing to smile after miscues. But at the first moment of any real tension, down 15-30 on her serve while trailing 4-3, Keys blinked, double-faulting twice.

Some terrific returning by Serena in the second set kept the heat on Keys.

"I think I served pretty well. I think she returned better," Keys said. "To beat Serena when she's playing well, you have to do a lot of things great."