Serena and Venus Williams will meet in the Rogers Cup semi-finals after each coming through gruelling quarter-finals in Montreal.
World number one Serena had to come from a set down to beat close friend Caroline Wozniacki - and then saw her sister follow the same path against Carla Suarez Navarro.
Serena Williams was broken in game five and lost the first set when she drilled a forehand into the net, with Wozniacki then opening up a 4-2 lead in the second and taking a 15-30 advantage on the top seed's serve.
But Williams held firm, broke twice to win the set and went on to complete a 4-6 7-5 7-5 win in two hours and 41 minutes.
"When you play someone like Caroline, you know your match might be a little longer than normal," she said. "You go in there thinking 'This is not going to be routine'."
Venus then stepped on court looking to follow up her hard-fought win over sixth seed Angelique Kerber on Thursday, and there were signs of a hangover from that outing in a collapse from 4-3 up in the opening set to a set and a break down.
But she responded well by winning 10 of the next 11 games to set up a 4-6 6-2 6-3 win in two hours and 11 minutes.
"I have to play well. That's pretty much it," Venus said of her 25th career meeting with her sister, who leads the head-to-head record 14-10.
"There's no secret or science to it. I think anyone who has gotten any wins against her, they've pretty much played the match of their life."
Serena added: "I definitely don't like playing her - I think I've lost to her more than anyone else on the tour. (But) I definitely feel great when I see her playing so well."
In the other semi-final, Agnieszka Radwanska will take on unseeded Russian Ekaterina Makarova aiming to get revenge for her Wimbledon elimination.
Third seed Radwanska exploited the travails of Victoria Azarenka - the eighth seed's heavily strapped right knee slowing her down badly, although an erratic performance had left her in trouble in any case.
The Pole broke three times in each set to win 6-2 6-2 in an hour and five minutes, with Azarenka virtually unable to break into a run from midway through the second set.
Makarova ended the run of American qualifier Coco Vandeweghe, who had accounted for former world number ones Ana Ivanovic and Jelena Jankovic in her previous two rounds.
The Russian broke serve three times in the first set and, after being pegged back in the second, stormed through the decider to win 6-1 4-6 6-1.