New York - Serena and Venus Williams advanced to a dramatic US Open quarter-final showdown with straight-set triumphs Sunday while Canada's Eugenie Bouchard withdrew from her fourth-round match due to a concussion.
World number one Serena Williams served well in moving within three match victories of a rare calendar Grand Slam with a 6-3, 6-3 triumph over US 19th seed Madison Keys.
Serena made only six unforced errors to 19 for Keys and hit 58 percent of her first serves. She took 79 percent of those points and 70 percent on her second serves, nearly twice Keys' second-try effectiveness.
"I'm so proud. I'm so glad it's so much better," Serena said of her serve.
"My only chance was to start out fast. I just stayed in there and was able to relax."
Venus, seeded 23rd, downed 152nd-ranked Estonian teen qualifier Anett Kontaveit 6-2, 6-1 at Arthur Ashe Stadium, matching her deepest Grand Slam run since reaching the 2010 US Open semi-finals.
"We're both prepared," Venus said. "Even though you're playing your sister you have to be prepared and focus. The preparation doesn't change."
Serena, who holds all four major tennis titles, is trying to complete the first calendar Grand Slam since Steffi Graf in 1988 and match Graf's Open Era record of 22 career Slam singles titles by capturing her Open Era record seventh US Open crown.
But Venus, two years older than Serena at 35, could play the spoiler as she tries to add to a trophy collection that includes the 2000 and 2001 US Opens and five Wimbledon titles, the most recent in 2008.
"I don't think anyone wants to be a spoiler. I think people love to see history being made," Venus said.
"But at the same time, you're focused on winning your match even though the circumstances are really much different than you."
Bouchard, who suffered a head injury after slipping in the locker room late Friday, pulled out of her match against Italy's Roberta Vinci with what US Open tournament director David Brewer said was a concussion.
Bouchard had pulled out of mixed and women's doubles on Saturday.
The 21-year-old from Montreal was a French and Australian Open semi-finalist last year and lost the 2014 Wimbledon final to Petra Kvitova.
Her head injury to a season of setbacks, including first-round French Open and Wimbledon exits and a 12-17 record.
Vinci advanced by walkover to her third US Open quarter-final in four years to match her deepest Grand Slam run.
She next faces the winner of a later match between Russian 13th seed Ekaterina Makarova and France's Kristina Mladenovic.
Serena leads the Williams sibling rivalry 15-11 but they have split four US Open meetings, including the 2001 and 2002 finals.
Serena won their most recent matchup in the fourth round at Wimbledon in July.
"Awesome. It has been awesome," Venus said when asked about her sibling rivalry.
"I'm so proud of Serena and I think she's proud of me. We inspire each other."
Serena won her first set in 27 minutes, Keys handing her the lone break in the sixth game.
Serena hit a backhand winner to break Keys for a 4-3 edge in the second set and broke again on match point when Keys, who also lost to Serena in the Australian Open semi-finals, double faulted to end matters after 68 minutes and ensure an all-Williams showdown.
"She is playing great," Serena said of Venus. "I have to play like I did today or better. But at least one of us, a Williams, will be in the semis, so that's good."
Venus, who lost the 1997 US Open final to Martina Hingis when Kontaveit was 21 months old, broke in the fourth game and again in the eighth, Kontaveit hitting a forehand long to drop the set after 24 minutes.
Kontaveit, who lost in the first round at the past two Wimbledons in her only prior Grand Slam appearances, swatted a forehand beyond the baseline to give Venus a 2-0 lead in the second set and sent a backhand long to surrender a break for a 4-0 deficit.
Williams held twice more, denying the European's lone break point in the final game to win in 50 minutes.
"Experience helped a lot," Williams said. "I was able to understand what it's like to play in this moment more than she was."
Kontaveit, 19, was thwarted in her bid to match the best US Open qualifier showing, American Barbara Gerken's quarter-final run in 1981.
Even with the loss, Kontaveit will nearly double her career prize money with $213 575 for her US Open run.US Open results on Sunday, the seventh day of the 2015 tournament at Flushing Meadows (x denotes seeded player):
Novak Djokovic (SRB x1) bt Roberto Bautista Agut (ESP x23) 6-3, 4-6, 6-4, 6-3
Feliciano Lopez (ESP x18) bt Fabio Fognini (ITA x32) 6-3, 7-6 (7/5), 6-1
Jo-Wilfried Tsonga (FRA x19) bt Benoit Paire (FRA) 6-4, 6-3, 6-4
Marin Cilic (CRO x9) bt Jeremy Chardy (FRA x27) 6-3, 2-6, 7-6 (7/2), 6-1
Serena Williams (USA x1) bt Madison Keys (USA) 6-3, 6-3
Venus Williams (USA x23) bt Anett Kontaveit (EST) 6-2, 6-1
Kristina Mladenovic (FRA) bt Ekaterina Makarova (RUS x13) 7-6 (7/2), 4-6, 6-1
Roberta Vinci (ITA) bt Eugnie Bouchard (CAN x25) - walkover