First Division side St Johnstone produced a superb performance to reach the last four of the Scottish Cup at the expense of their SPL opponents.
Peter MacDonald gave Saints a deserved lead
Saints took the lead after 20 minutes when Peter MacDonald's looping shot from a William McLaren cross beat Well keeper Graeme Smith.
Jason Scotland, who was dangerous throughout, headed Saints' second in the 72nd minute.
Ross McCormack pulled one back with a header but Saints deservedly held on.
Motherwell winger Darren Smith failed to recover from an ankle injury and was replaced by Steven McGarry in the only change from the side which drew at Love Street.
Saints also made one change with midfielder Martin Hardie returning from a ban to replace the suspended Derek McInnes.
Allan McManus recovered from a groin injury to take his place in defence.
St Johnstone started the brighter of the two sides with striker Scotland proving a thorn in Motherwell's side in the opening 10 minutes.
MacDonald gave St Johnstone a deserved lead in the 20th minute when he lifted the ball over Motherwell keeper Smith from McLaren's cross.
Saints looked likely to add to their lead with Hardie, who was booked moments earlier, going close with a header from Goran Stanic's excellent cross.
McCormack's shot in the 39th minute was one of the few occasions in the first half that Motherwell threatened.
St Johnstone were the better side for long periods
The home side's Scott McDonald almost fashioned an equaliser a minute later, but his rasping drive from the edge of the area was deflected wide for a corner.
In the second half the visitors looked the likelier to score and Scotland sent the Saints fans wild when he headed home Stanic's cross from close range with 20 minutes left.
Simon Mensing could have made the tie safe two minutes later when he pounced on a misplaced pace but fired wide from 20 yards.
Smith and Motherwell captain Stephen Craigan had a heated exchange with nine minutes remaining, when the latter's pass-back almost gifted MacDonald his second goal of the evening.
McCormack headed home McDonald's cross with six minutes remaining to spark the home fans into life and set up a tense finish.
Smith made a number of brilliant saves to prevent a heavier defeat for Well.
St Johnstone substitute Andy Jackson spurned a great chance to make it 3-1 - but the visitors held on to ensure their place in Thursday's draw for the semi-finals.
Motherwell: Graeme Smith, Corrigan, Craigan, Reynolds, Murphy (Clarkson 73), McGarry (Molloy 58), Kerr, Fitzpatrick (Keegan 82), Paterson, McCormack, McDonald.
Subs Not Used: Martin, Quinn.
Goals: McCormack 85.
St Johnstone: Cuthbert, Lawrie, McManus, Mensing, James, Stanic, McLaren (Morais 83), Hardie, Sheerin, MacDonald (Milne 90), Scotland (Jackson 89).
Subs Not Used: Halliwell, Dyer.
Goals: MacDonald 21, Scotland 72.
Ref: M McCurry.