Darren Ferguson's men saw their hopes of a second successive move up the Football League ladder dealt a blow as Yeovil swooped for a fourth straight success.
Posh were undone by two set-piece goals in the space of three first-half minutes as Paul Warne (31) and Aaron Brown (33) made the most of delightful Andy Welsh deliveries.
Posh could not find a way through when launching an attacking onslaught late in the opening period, Aaron Mclean seeing a stoppage-time header cleared off the line by Jon Worthington seconds after guiding a low shot inches wide of the far post.
Mclean refused to be denied, though, with his 15th goal of the season finally coming 11 minutes into the second half thanks to a classy half-volley after some fine work by George Boyd.
Mclean and Boyd both went close to levelling the scores but Posh hopes of completing the fightback were undone in the 69th minute when Lewis contrived to fumble a weak shot from Shaun MacDonald into his own net.