Opening half tries from Tommy Bowe and Ugo Moyne - this one created by Bowe - and eight points from the boot of Stephen Jones had the Lions 18-12 in front at the break.
Willem de Waal closed the gap with an early second half penalty, moments after he's come up short with one, but Martyn Williams rounded off a period of Lions pressure with a try after good work by Donncha O'Callaghan .
De Waal was again on the mark with his fifth successful effort in the 59th minute and Pietersen's try four minutes later tied the match going into the final quarter.
Lions: Rob Kearney (James Hook 62 min), Tommy Bowe, Keith Earls, Riki Flutey, Ugo Monye, Stephen Jones, Harry Ellis, Andrew Sheridan, Matthew Rees, Phil Vickery (captain)(euan Murray), Donncha O'Callaghan, Nathan Hines, Joe Worsley (Tom Croft 69 min), Martyn Williams, Andy Powell. Replacements: Ross Ford, Simon Shaw, Shane Williams, Gordon D'Arcy.
Western Province: Joe Pietersen, Tonderai Chavhanga, Morgan Newman, Peter Grant, Gcobani Bobo, Willem de Waal, Dewald Duvenhage, Luke Watson (c), Duane Vermeulen, Pieter Louw, Anton van Zyl, Martin Muller, Brok Harris, Tiaan Liebenberg, Wicus Blaauw. Replacements: Hanyani Shimange, JD Moller, De Kock Steenkamp, Zandre Jordaan, Conrad Hoffmann, JJ Engelbrecht, Gio Aplon
Referee: Mark Lawrence (South Africa)