Match ends, Motherwell 1, Dundee United 2.
Dundee United exacted a measure of revenge against Motherwell by coming from behind to earn a place in the Scottish Cup last 16.
Last season's beaten finalists, who lost in the league at Fir Park in early November, trailed to Henrik Ojamaa's arrowed seventh-minute opener.
After fine saves by Dan Twardzik in the Motherwell goal, United levelled with a 66th-minute John Souttar header.
And they clinched the tie eight minutes from time when Keith Watson nodded in.
It was a merited triumph for United but it was still possible to feel some sympathy for Motherwell.
Having faltered at this stage of the competition exactly a year ago against Albion Rovers, a second victory over United at Fir Park this month would have been a welcome tonic for the managerless Lanarkshire side.
And, for long spells, it looked as if they might progress.
Caretaker Kenny Black's decision to play three in central midfield meant his side enjoyed the better of the first half, their numerical advantage in the middle of the park limiting United's ability to build play.
Emboldened by Ojamaa's fine goal, which the Estonian forward hit past Radoslaw Cierzniak from 20 yards after accepting Paul Lawson's pass, Motherwell played with a confidence lacking in recent performances.
Josh Law should have doubled their advantage when Zaine Francis-Angol's delivery found him unmarked six yards from goal but, that aside, clear openings were scarce for the hosts.
However, the interval introduction of Calum Butcher stiffened the United midfield and they soon wrested control, only to spurn a series of opportunities before finally breaching the exemplary Twardzik.
The Czech custodian had dashed from his line to thwart Ryan Dow, Gary Mackay-Steven, Butcher and Stuart Armstrong, who had each burst clear.
But he was let down by his defenders for both of United's goals.
First, Souttar was given too much space in the six-yard box and met Armstrong's corner with a well-directed header into the net. Then Conor Townsend's cross was met by the unattended Watson, who was a late replacement for Jaroslaw Fojut.
Other opportunities came and went for Jackie McNamara's side. Armstrong squirted wide after an intricate exchange with Chris Erskine, and the latter had three efforts from distance - one deflected wide, one over and a third pawed clear by the alert Twardzik - as United began to ratchet up the pressure.
It soon told, Souttar's goal breaking Motherwell's defiance and making Watson's winner almost inevitable.
- 17Francis-AngolSubstituted forKerrat 84'minutes
- 8LawsonSubstituted forReidat 48'minutes
- 14LasleyBooked at 33mins
- 11VigursBooked at 20mins
- 18LawSubstituted forErwinat 76'minutes
- 24OjamaaBooked at 71mins
- 3TownsendBooked at 38mins
- 6PatonBooked at 20mins
- 21TelferSubstituted forButcherat 45'minutes
- 17ErskineSubstituted forAdeyemoat 84'minutes
- 11Mackay-StevenSubstituted forConnollyat 62'minutes
- Craig Thomson
- Shots on Target
Second Half ends, Motherwell 1, Dundee United 2.
Attempt missed. John Sutton (Motherwell) header from the centre of the box is just a bit too high.
Corner, Dundee United. Conceded by Simon Ramsden.
Keith Watson (Dundee United) wins a free kick in the attacking half.
Foul by Henrik Ojamaa (Motherwell).
Foul by Iain Vigurs (Motherwell).
Paul Paton (Dundee United) wins a free kick on the left wing.
Calum Butcher (Dundee United) wins a free kick in the attacking half.
Foul by Craig Reid (Motherwell).
Substitution, Dundee United. Ola Adeyemo replaces Chris Erskine.
Substitution, Motherwell. Fraser Kerr replaces Zaine Francis-Angol.
Goal! Motherwell 1, Dundee United 2. Keith Watson (Dundee United) header from the centre of the box to the bottom right corner. Assisted by Ryan Dow.
Corner, Motherwell. Conceded by Conor Townsend.
Attempt saved. Stuart Armstrong (Dundee United) right footed shot from outside the box is saved in the centre of the goal.
Substitution, Motherwell. Lee Erwin replaces Josh Law.
Foul by Mark O'Brien (Motherwell).
Ryan Dow (Dundee United) wins a free kick in the defensive half.
Attempt saved. Ryan Dow (Dundee United) right footed shot from the right side of the box is saved in the bottom left corner.
Henrik Ojamaa (Motherwell) is shown the yellow card.