While Saints hammered Wakefield Wildcats 40-16 to claim their 18th win of an impressive campaign on Friday, Leeds fell to third 48 hours later with a stunning 40-36 loss at bottom club London Broncos, who claimed their first win of the campaign.
Having trailed 30-8 early in the second half, the Broncos produced a superb fightback to seal their maiden success of a tortuous season, stand-off Joe Keyes' third try seven minutes from time effectively sealing victory and sending Leeds into next Saturday's Challenge Cup final at Wembley on a disappointing note.
The result leaves Leeds four points adrift in third, and Wigan are just a point further ahead after they were held to a 14-14 draw by Hull KR on Thursday.
The defending champions needed a late rally to salvage a share of the spoils in East Yorkshire, with two tries in the final eight minutes from Sean O'Loughlin and Josh Charnley - the second from the England winger in injury time - earning the visitors a draw.
Matty Smith even had the chance to seal maximum points for Wigan, but he missed with his conversion from the touchline following Charnley's score.
Elsewhere, Warrington Wolves and Huddersfield Giants both gained ground in the fight for second place and are now among the three teams level with Leeds and a point behind Wigan.
Tony Smith's Warrington crushed Castleford Tigers - Leeds' opponents in the cup final - 48-10 to leapfrog Tigers into fourth, while Huddersfield saw off a spirited challenge from Widnes Vikings to claim a 28-14 success.
The already relegated Bradford Bulls continued their recent upturn in fortunes by beating Hull FC 34-28, while Salford Red Devils kept up their slim play-off hopes as they overcame Catalan Dragons 34-22 courtesy of 14 points from Kevin Locke.