Venus Williams will play Agnieszka Radwanska in the final of the Toronto Masters after rallying to take a three-set win over sister Serena 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.

She will meet Radwanska in Sunday’s final after the third seed from Poland defeated unseeded Russian Ekaterina Makarova 7-6 (7-1) 7-6 (7-3).

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.”

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 said.

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-head series 14-11.

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.

“If I am fortunate enough to win this tournament, then I am sure it will be dance-worthy,” Venus said.