NEW YORK » No reason 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 during this U.S. 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 U.S. Open, Serena is trying to become the first tennis 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 U.S. 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 U.S. 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."

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 the winner of Sunday night’s match between No. 13 Ekaterina Makarova and 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.

Djokovic lost a set for the first time this tournament — and responded by stomping on a racket while it leaned on his red bag next to a changeover chair, earning a warning from the chair umpire. But he reached the quarterfinals at a 26th consecutive major by beating 23rd-seeded Roberto Bautista Agut 6-3, 4-6, 6-4, 6-3 at night.

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."