When referee Graham Horwood finally restored order he booked Evans for the foul and home defender Chris Westwood and Wrexham striker Drewe Broughton for their parts in what followed.
Lewis, last month's £400,000 Posh record signing from Norwich, had to be carried off on a stretcher with manager Darren Ferguson forced to blood teenage reserve keeper James McKeown for the last 20 minutes.
McKeown managed to keep his goal intact, aided by a glaring miss by Evans who blazed wide in the closing minutes.
Fielding just two players from the side booted out of the FA Cup by Posh in November, Wrexham survived early pressure in which Craig Mackail-Smith twice failed to capitalise on promising situations.
Wrexham, bidding to continue a run in which they defeated promotion contenders MK Dons and Darlington, grew in confidence with Danny Sonner increasingly dominating play from midfield.
Lewis had to be on his toes to snatch away a shot from Chris Llewellyn, twice a recent Posh transfer target, before Phil Bolland powered a header narrowly wide and Lewis was again tested by Llewellyn.
The closest call for Posh came when Stuart Nicholson saw a header smack against Lewis' right-hand post with Craig Morgan sweeping the rebound to safety.
Posh's Aaron McLean at times looked like a midget in a land of giants as Bolland and Evans commanded the Wrexham penalty area and when the 25-goal marksman did get the ball after fine work by George Boyd on the left, a badly scuffed shot held no terrors for Gavin Ward.
Mackail-Smith looked to have snatched the points when bundling the ball into the Wrexham net deep into stoppage time only to see Morgan penalised for pushing in the build-up.