Serena Williams of the US makes a sign towards fans after defeating compatriot Madison Keys in their fourth round match at the US Open Championships tennis tournament in New York, September 6, 2015. [Photo/Agencies]

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 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 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 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.