But Liam Craig slotted home in the last minute to give the visitors victory.
Smith punched Liam Craig's corner into his own net after 36 minutes to gift St Johnstone the lead.
Goals from Liam Craig and Cillian Sheridan put Saints into a commanding lead within the first 10 minutes.
Liam Craig's delivery was perfect for Grainger whose powerful volley flashed past a ruck of players but not Langfield.
The Saints took an 18th minute lead when Sam Parkin headed in Liam Craig's cross from wide on the left.
Liam Craig forced MacDonald to parry a ferocious free-kick over the crossbar after Andy Webster had hauled back Francisco Sandaza 25 yards out.
Peter MacDonald burst through the United defence to give St Johnstone a sixth-minute lead and Liam Craig got the visitors second on 21 minutes.
Ryan Stevenson failed to find the target with an early chance for Hearts, while Saints' Liam Craig forced a good save from Jamie MacDonald.
Steve Lomas' side kept up the pressure and Chris Millar floated a cross over from the right that was won in the air by Liam Craig.
Alan Maybury sent an effort over the bar shortly after the hour mark before Liam Craig headed in a Danny Grainger corner after 73 minutes to level.
Soon after, a Liam Craig volley went straight into the chest of Jaakkola and the Finnish keeper was forced to tip away a Dave Mackay free-kick with only 13 minutes gone.
Wardlaw's afternoon grew more frustrating when he was booked by referee John McKendrick after being caught by a dangerous high challenge from Liam Craig, with the free-kick going the home side's way.
The start of the second period should have been marked by an equaliser by Kenny Deuchar, who connected with an accurate ball from Gary Irvine that eluded Liam Craig but the forward shot with insufficient venom to bother Langfield.
But the Saints goalie struggled with a few first-half corners, flapping at Dougie Imrie's first delivery and relying on Liam Craig to head off the line as Martin Canning's looping header looked set to sneak in at the post.
St Johnstone were presented with a real chance in the 28th minute when Duberry's ball was only half-cleared by Ruben Palazuelos with goalkeeper Marian Kello drawn out of position behind him, but Liam Craig volleyed wide of the empty net.
Davies will emulate midfielder Craig Gardner and defender Liam Ridgewell as former Villa players plying their trade across the city.
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