Venus Williams upset sister Serena to reach a Rogers Cup final against Agnieszka Radwanska in Montreal.
Third seed Radwanska was at her inventive best to see off the dogged Ekaterina Makarova in two tie-breaks - but the main story was always going to be the battle of the siblings.
World number one Serena won the first set on a tie-break but Venus found her top form to win the next two in style, in a match screened live on BT Sport.
A mishit helped Venus force a first-set tie-break in a superb rally which ended with Serena fizzing a cross-court forehand just wide - but it was the younger sister who took the set as Venus sent a backhand long.
Venus broke at the first attempt in set two and levelled the match comfortably as Serena dumped a ball tamely into the net.
Venus won a backhand exchange to go up a break at 4-2 in the decider and a huge serve wrapped up a 6-7 (2/7) 6-2 6-3 win lasting just over two hours - closing the career head-to-head series to 14-11 in Serena's favour.
Venus told wtatour.com: "The level of game we played was pretty high. We're both serving well, returning well, moving well - everything is happening so well that all of our matches seem to be very competitive."
Serena added: "She was really opening the court well. She's such a good mover. She moves really well.
"I think her serve was way more consistent than mine. I had more aces, but she served better. Ultimately I think she returned better as well."
Venus Williams will go for her 46th WTA title in her 74th final when she takes on Radwanska, whose semi-final was a set shorter but two minutes longer.
Makarova saved four break points to take the first set the distance but had no answer in the breaker, capitulating 7-1.
Radwanska led by a break at 3-1 in the second but her Russian opponent hit back to lead 4-3.
The Pole had a huge stroke of luck as a huge net-cord wrong-footed her opponent to set up a break point which she took to leave herself serving for the match - but again she let the advantage slip.
She was taken to another tie break and saw an early 4-1 lead pegged back but a final surge took her to a deserved 7-6 (7/1) 7-6 (7/3) win.
"It was a great match," Radwanska said. "It was two hours of fighting with a lot of long rallies and long games.
"I had to run a lot to win that match, she's a very consistent player and she doesn't give you any free points."
Looking ahead to the final against Williams, Radwanska added: "She's really on fire, she's playing amazing this week. It's not going to be easy, for sure."