Venus Williams upset little sister Serena 6-7(2) 6-2 6-3 on Saturday to register the first win over her top ranked sibling in five years and move into the Rogers Cup final in Montreal.
The meeting marked the 25th time the sisters had squared off but the first time Venus had come out on top since a 2009 semi-finals encounter in Dubai.
World number one Serena holds a 14-11 lead in their head-to-heads after coming out on top in their previous five encounters, including the 2009 Wimbledon final.
Clashes between the pair, which at one time were an almost regular and welcome occurrence on tennis courts around the world, have become rare and their meeting in Montreal was just the second since the 2009 Tour championships.
The siblings have won 106 WTA Tour singles titles, including 24 grand slams, between them.
"It was a tough match out there," said Venus, who will
meet Poland's Agnieszka Radwanska in Sunday's final. Third seed Radwanska beat Ekaterina Makarova of Russia 7-6(1) 7-6(3) to advance to her second final of the year.
"I don't think we played for a number of years, as well. It's not like we've been playing year in and year out," added Venus.
"I definitely expected a tough match. She played well. She hit so many aces.
"What's so unique about the situation is that we're both very good players.
"I think typically you may have some siblings, one is quite good, one is not as good, so you kind of know what the result is, or the one that's better knows they're going to win.
"I think we both know when we walk out there, it's not like you're guaranteed a win. I think that's what makes it challenging for both of us."
While a Williams-Williams matchup is one tennis fans have always looked forward to, for the sisters it is something they could live without.
There was little excitement when the match was over, the two players walking slowly to the net and hugging briefly.
Venus's reaction at ending her sister's Rogers Cup title defense was muted as she turned and offered a small wave to the crowd before slumping into her courtside chair and staring blankly across the court.
If losing to her sister stung Serena, she hid it well.
"For me it was just a regular match," she shrugged. "Obviously she's my sister but I still want to go out there and try to play well and try to win.
"I think Venus played a great match. It was what it was.
"Obviously I always want her to do the best that she can. I want to do the best that I can, too.
"I just got to do better, that's all."
In the other semi, neither player broke serve in the opening set, but Radwanska broke twice in the second to serve for the match at 5-4.
However, she was broken a second time herself and the set went to a tiebreak, where Radwanska prevailed to gain revenge for her fourth round defeat by Makarova at Wimbledon.
"I think it was a great match, two hours of fighting and long rallies and tight games," she said.
"I had to really run a lot to win that match because she's so good. I think I was just better on a few points today."
(Reporting by Steve Keating in Toronto. Editing by Gene Cherry/Rutherford)