Motherwell reached the Scottish League Cup semi-finals for the first time in six years as Lennon Miller's stoppage-time penalty knocked out Dundee United.

Summer signing Zach Robinson's howitzer two minutes into first-half added time turned the game in the hosts' favour before Louis Moult stunned his former side with an equally-superb 83rd-minute finish.

But 18-year-old Lennon gave the hosts victory after Moses Ebiye was felled by Luca Stephenson's rash challenge.

An early Kristijan Trapanovski shot straight at Motherwell goalkeeper Aston Oxborough from an acute angle looked to be all either side had to offer in the first half - until Robinson's moment of brilliance.

United had edged what had drifted aimlessly into a dull stalemate, but the Motherwell striker pounced after Miller's shot on the turn was parried by Jack Walton and thundered it past the goalkeeper from 14 yards.

Motherwell were now the more dangerous side and Emmanuel Adegboyega denied Miller twice within seconds, beating the midfielder to a dangerous ball into the six-yard box, then clearing his shot off the line from the resulting corner.

United made changes and their fortunes improved.

Defender Paul McGinn was fortunate to only earn a yellow card for an elbow-first, last-man challenge on Moult.

Then fellow substitute Sam Dalby should have done better than fire at Oxborough's legs when put through by Moult.

However, when the roles were reversed, the latter's sublime volley levelled the game.

Walton pushed away a Ebiye header from point-blank range but was beaten after the substitute striker earned the deciding spot kick that took Motherwell into the last four for the first time since they lost to Celtic in the 2018 final.

Motherwell manager Stuart Kettlewell had named an unchanged side, sticking with Robinson up front despite Ebiye taking his total to four for the season after back-to-back goalscoring cameos off the bench.

It looked like it was a strategy playing into United's hands as the visitors forced the hosts to play long, with Robinson all too often outmuscled by former Fir Park centre-back Declan Gallagher and Ross Graham.

However, it paid off on the stroke of half-time as the summer arrival from Wimbledon broke his season's duck in spectacular fashion.

It looked like that would be enough to secure victory until Moult stunned his former side in a hectic finish lapped up by an impressive crowd of more than 8,000.

Ebiye, though, was to prove the super sub again to hand much-heralded teenager Miller the chance to again show his immense promise and cool head from the spot.

United boss Jim Goodwin had handed Ross Docherty his first start of the season, the captain replacing Kevin Holt in midfield - as he did before half-time in Sunday's league defeat by Rangers.

Goodwin cited the former Partick Thistle 31-year-old's composure on the ball for that switch and it paid off early as United looked the more creative force.

However, there was a lack of threat up front, while David Babunski and Kristijan Trapanovski, the summer signings who have impressed so far this season, failed to live up to the occasion before both were withdrawn.

The changes hauled United back into the game, with on-loan Wrexham striker Dalby looking dangerous before chesting down for Moult's equaliser.

However, the visitors had left it too late to come to life and, after that late sucker punch, will look back on this as an opportunity missed to reach their first semi-final since losing to Celtic in the 2015 final.

Motherwell manager Stuart Kettlewell: "The supporters have to enjoy it. They made this place special tonight. The atmosphere was unbelievable.

"The nerve and composure of an 18-year-old [Miller] to take that penalty. I don't think many of us in the stadium doubted he would put that in the back of the net.

"It's a special moment for him and the club."

Dundee United manager Jim Goodwin: "This is as low as I have felt after a game for a long period of time.

"Second half, I thought we were really good. We had done well to get back into it. It's a great goal, a brilliant finish into the top corner.

"Unfortunately, it's a poor decision in a key moment in the latter stages of the game that cost us."