St Mirren secured their first win of the season to push Hearts deeper into relegation trouble and ease the pressure on Buddies boss Danny Lennon.
John MGinn headed a powerful opener and Paul McGowan stabbed the second over the line after a parried clearance from goalkeeper Jamie MacDonald.
Hearts piled on the pressure, but the visitors always looked like holding on.
And the Paisley club extended the gap between themselves and the Premiership's bottom side to 12 points.
It ended St Mirren's run of eight games this season without a win and burst the pre-match optimism around Tynecastle after they had already reduced the 15-point penalty imposed for going into administration.
And it justified Lennon's decision to name an unchanged side after having changed three of his four defenders and his goalkeeper for Monday's draw against Aberdeen.
A cup-tie atmosphere and a healthy crowd of 14,769 was ensured for a match talked up by both sides as a vital game even this early in the battle to avoid the drop.
Hearts' loyal support, who have done so much to keep the club in business through administration with their fund-raising, arrived in expectation of delivering a killer blow to Lennon's troubled time as St Mirren manager.
The packed stands will have been a daunting sight for the visiting side and their inspired young opponents soon forced them back on their heels.
Chris Dilo leaped athletically to tip over a Danny Wilson header, the goalkeeper was also tested by Callum Paterson from long range, while the fit-again Ryan Stevenson fired into the side netting.
However, uncertainty within Hearts' back-line overshadowed their lively forward play and breathed confidence into fluttering Buddies hearts.
MacDonald's chest came to the rescue after Dylan McGowan's short pass-back presented Kenny McLean with a one-on-one with the goalkeeper.
McLean flashed a 25-yarder just wide then McGowan pounced on another moment of hesitancy to send a lob over the goalkeeper and just past the post.
McGowan should have done better than fire over after cleverly finding room for himself on the edge of the penalty box, while McLean had a curling free-kick beaten away by MacDonald.
Paterson's lack of composure led to a good opening ending high in the stand before the Buddies broke through at the other end.
Conor Newton's curling cross found McGinn powering in at the back post to head into the roof of the net.
Hearts were first out the traps again after the break and Paterson once more failed to hit the target when found in acres of space only 16 yards from goal.
St Mirren breathed another sigh of relief before marching down the other end to extend their lead.
MacDonald could only parry David van Zanten's low cross-come-shot into a crowded six-yard box and McGowan looked like he knew little about it as it struck his shin before trundling over the line.
Apart from a McLean header tipped over by MacDonald, St Mirren were camped in their own half thereafter.
For all their pressure, with Jamie Walker leading the way with some jinking runs into the box, Hearts were unable to avoid their own run without a win extending to seven games.
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