Venus Williams booked her spot in the Toronto Masters final by rallying to take a three-set win over sister Serena on Saturday in their 25th career meeting.

Seven-time grand slam winner Venus benefited from nine Serena double faults en route to a 6-7 (2-7) 6-2 6-3 victory at the $US2.5 million ($A2.70 million) hardcourt tournament in Montreal.

Venus posted her first win over top seed and defending champion Serena since another three-set victory five years ago in Dubai.

“Serena is at her top level,” Venus said. “You can’t win every match. You can’t win them all.

“She’s number one. Nobody wants to play her. If you do, you have to play the match of your life. There is no doubt about that.”

Saturday was also their first meeting since Serena thrashed her older sibling 6-1 6-2 last year in Charleston.

The contest lasted just over two hours as Venus hammered six aces and had just two double faults.

“Big sister taught little sister a lesson…,” world No.1 Serena wrote on her Twitter account.

Many of the crucial points in the semi-final came off second serves as Venus won 56 per cent of hers compared to just 36 percent for Serena, who still leads their head-to-heads 14-11.

In Sunday’s final, Venus will face either third-seeded Agnieszka Radwanska of Poland or unseeded Russian Ekaterina Makarova.