Venus Williams booked her spot in the Montreal WTA 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 hardcourt tournament.

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

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

“Big sister taught little sister a lesson…,” world number one Serena wrote on her Twitter account.

The 25th career meeting between the two lasted just over two hours as Venus hammered six aces and had just two double faults.

Many of the crucial points in the semi-final came off second serves as Venus won 56 percent of her second serves 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.

The 34-year-old Venus became the oldest player to reach the final of this event in a quarter of a century as she guns for her 46th career singles title.

This also marked the first time Venus and Serena appeared on the same court together since Serena’s infamous wobble at Wimbledon.

Six weeks ago Serena exited Wimbledon in bizarre fashion. Appearing dizzy and bewildered during a Wimbledon doubles match with her sister, she struggled just to bounce the ball and then double-faulted four times in a row. Serena later blamed her performance on a virus.

Serena, a three-time champion at the event and who was coming off a victory last week in Stanford, fired 19 aces but couldn’t overcome her double faults and poor second serves.