Saints' top-four hopes are hanging by a thread after Leicester Tigers secured their seventh successive east midlands derby success on Saturday afternoon.
Tigers' replacement fly-half Owen Williams held his nerve with two penalties late on as he presented his team with a vital 36-31 win at Franklin's Gardens.
Williams was even named man of the match as he showed his class after coming on for Freddie Burns at half-time.
It was a bitter pill for Saints to swallow as they had led 22-16 at the break thanks to tries from Nafi Tuitavake, Ahsee Tuala and Louis Picamoles.
Leicester replacement flanker Mike Williams was sin-binned with 12 minutes to go and Stephen Myler had the chance to put Saints eight points up with a penalty.
But he missed and 14-man Tigers roared back, scoring through Lachlan McCaffrey and then finishing the job with Owen Williams' ice-cool kicking from the tee.
It means Leicester move into the top four, above Bath, who play at Saracens on Sunday, and Saints are now six points behind their rivals in the Aviva Premiership standings with four games of the regular season to go.
It had all started so well for Jim Mallinder's men on a sun-drenched Gardens afternoon as Tongan centre Tuitavake took advantage of Tigers full-back Telusa Veainu's misjudgement to score.
Myler converted to make it 7-0 after just three minutes, but Tigers responded almost immediately.
Adam Thompstone had the freedom of Franklin's Gardens as he wondered in for an easy try, with Burns converting easily to level the scores.
But in what was already a frantic game, it didn't take long for Saints to get back in front with another score of their own.
The ever-impressive Picamoles stormed down the right and offloaded perfectly to the onrushing full-back Tuala, who cruised in for the score.
Myler again converted to make it 14-7, but Burns quickly narrowed the gap with a penalty earned at a five-metre scrum.
Tigers won another scrum penalty soon after and Burns again did the business as the game got even tighter.
But Picamoles was the first-half's stand-out performer and he was endearing himself more to the home fans minute by minute.
There were cries of 'I love you, Louis' when the Frenchman burst over the line to score Saints' third try.
Myler missed the conversion and Burns quickly landed a penalty in what was turning into another east midlands classic.
There were offloads all over the place in a frenetic game, with Saints sharp in attack, and Mallinder's men won a penalty before the break, with Myler slotting it.
Saints thought they had grabbed the bonus-point score in the final play of the first half as Christian Day stormed over in the corner.
But lock's foot was deemed to be in touch and the players headed off with Tigers still just six points down.
The away side were forced to make two changes at the break, with fly-half Burns and flanker Brendon O'Connor taken off.
But it didn't affect Tigers and they were to score first in the second period as Ben Youngs found space down the left and sprinted in to dot down.
Williams converted to give his team a one-point advantage, but Myler quickly put Saints back in front with a penalty from in front of the posts.
And Myler was soon adding another three points after Tigers infringed trying to defend their own line.
But the scrum was still proving something of a struggle for Saints and referee Greg Garner gave Tigers another penalty, which was missed by Owen WIlliams.
Tigers replacement Mike Williams was soon heading for the sin bin after a smashing Teimana Harrison high at the breakdown.
But Myler failed to land the penalty that would have put his team eight points clear with 12 minutes to go.
And Tigers made him pay, roaring back with McCaffrey's try and Owen Williams' conversion which put the away side 30-28 up with nine minutes remaining.
Myler put Saints back in front with a fine penalty, but again Leicester were quickly back in their stride and they soon went in front again, with Owen Williams slotting a penalty.
Myler kicked the restart out on the full and Mike Williams returned to add to Saints' misery with three minutes left.
And Owen Williams rubbed salt in the wounds as he kicked another penalty to earn a five-point s win for the jubilant away team.
Saints: Tuala; Foden, Tuitavake (Burrell 71), Mallinder, North; Myler, Groom (Dickson 61); A Waller (E Waller 69), Hartley (Haywood 57), Brookes (Hill 61); Lawes, Day (Harrison 61); Wood (c), Clark, Picamoles (Gibson 69).
Leicester Tigers: Veainu; Thompstone, Tait, Mermoz, Pietersen; Burns (O Williams 40), B Youngs; Genge, T Youngs (c), Cole; Barrow, Kitchener (M Williams 65); Fitzgerald, O’Connor (McCaffrey 40), Hamilton.
Referee: Greg Garner