No need for any extra practice after this performance by Serena Williams.

Plus, it's not as if she needs to study too hard to figure out how to deal with her next opponent.

Playing the best she has in the US Open as she chases a calendar-year grand slam, Williams set up a quarterfinal against older sister Venus by making only six unforced errors in a 6-3, 6-3 victory over 19th-seeded Madison Keys on Sunday.

Looking ahead, No 1 Serena described No 23 Venus this way: "It's like playing a mirror."

Serena took only 68 minutes to dismiss Keys, a 20-year-old American with formidable serves and forehands who simply was outplayed.

Already a winner of the past four major tournaments, including last year's US Open, Serena is trying to become the first player to win all four in the same season since Steffi Graf in 1988.

Asked why it's been so long since anyone else pulled off a true grand slam, Serena paused for effect, smiled and answered: "I don't know why it took me so long."

Standing in the way at the moment is Venus.

This will be the 27th all-Williams match over their long and successful careers, and Serena leads 15-11, including 8-5 at majors.

Each has beaten the other twice at the US Open, with Venus winning the 2001 final and Serena the 2002 final.

"It's about, of course, forgetting that she's playing Venus," said Serena's coach, Patrick Mouratoglou.

Venus, at 35 the oldest woman in the tournament, was on court even less time than her sibling on Sunday, overwhelming 19-year-old qualifier Anett Kontaveit of Estonia 6-2, 6-1 in 50 minutes.

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 before her second-round match, when she double-faulted 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 a player 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."

Another women's fourth-round match scheduled for Sunday was scratched when 25th-seeded Eugenie Bouchard withdrew with a concussion, two days after slipping and falling in the locker room. That allowed Roberta Vinci of Italy to advance to the quarterfinals against Kristina Mladenovic.

Two men's matchups in the round of eight are set: No 1 Novak Djokovic vs No 18 Feliciano Lopez, and defending champion Marin Cilic vs No 19 Jo-Wilfried Tsonga.

Five top sibling showdowns

1998 Australian Open, 2nd round

Nowhere near a classic - it was a described as 'sub-par' by one newspaper - but noteworthy as it was the first meeting between the two sisters. Both appeared sporting beads in their hair. Venus, 17 at the time, triumphed 7-6 (4), 6-1 which was expected as she was ranked No 16 in the world while 16-year-old Serena was No 53.

2003 Australian Open final

One of the most dramatic finals between the sisters came in the heat of Melbourne, when Serena clinched the last Grand Slam title to elude her with a tension-filled 7-6 (4), 3-6, 6-4 victory.

It was only the sixth time a woman held all four of Grand Slams at the same time, and the first since Steffi Graf in 1994.

2008 Wimbledon final

Venus clinched her fifth Wimbledon title with a 7-5, 6-4 win in one of the few really high-quality clashes between the pair at the All England Club.

Venus, 28, defied blustery conditions to become one of only four players to lift the trophy five times or more in the modern era. She sealed the historic victory when Serena fired a backhand wide, but the celebration was muted as the sisters hugged each other across the net.

2009 Dubai semifinal

Venus beat top-ranked Serena 6-1, 2-6, 7-6 (3) to take a 10-9 lead in the sibling rivalry.

It was Venus' fifth win in their last seven meetings but Serena earned plenty of plaudits for defying the pain of a knee injury that had forced her to retire from a tournament the previous week.

It was only the second time a set had gone all the way to a tie-break in their 19 career meetings to that point.

2009 Wimbledon final

Serena ended Venus's two-year reign as Wimbledon champion with a 7-6 (3), 6-2 victory that secured her a third All England Club crown.

It was the fourth time the sisters had met in a Wimbledon final, with Serena, who now held the Wimbledon, Australian and US Open crowns, winning three of those showdowns.

The 27-year-old's 11th career win over her sibling regained the Wimbledon title she won in 2002 and 2003, and shattered five-time champion Venus's hopes of lifting the trophy for a third successive year.