Richie Foran scored with a deflected shot in the fifth minute of added time to earn Inverness a replay in a 3-3 thriller at Ross County.
Billy McKay had put the visitors ahead at the Global Energy Stadium on the half-hour only for Rocco Quinn to restore parity four minutes into the second half.
Foran put Caley back in front with 16 minutes to go but County were level again when Iain Vigurs netted with four minutes left. Richard Brittain put the home side ahead for the first time in the second minute of added time but Foran, the Inverness captain, ensured the spoils were shared.
Aberdeen and Motherwell will have to do battle again as Niall McGinn continued his potent form in front of goal this season as he headed in a last-minute equaliser to earn a 1-1 draw at Pittodrie.
It had, for the most part, been an even match between the two SPL sides although the Steelmen looked to be heading through thanks to Jamie Murphy's superb strike with 10 minutes remaining before McGinn levelled the scores.
St Mirren progressed with a comfortable 2-0 victory over Brechin City thanks to goals from Kenny McLean with a well-struck free-kick just after the half-hour mark and Jon Robertson four minutes before half‑time.
Johnny Russell's hat-trick and two goals from Jon Daly secured a comfortable 5-0 win for Dundee United at Stranraer, while Kilmarnock saw off Queen of the South 2-1 thanks to goals from Cillian Sheridan and Borja Pérez, with Nicky Clark on target for the visitors.
Forfar progressed to the next round after a 2-1 win over Ayr, while first-half goals from Steven Milne and Ryan Conroy gave Dundee a 2-0 win at Livingston.
Dunfermline won 1-0 at Partick Thistle thanks to a ninth goal of the season for Andy Barrowman, Raith Rovers beat Deveronvale 2-1, Falkirk were 1-0 winners at Stenhousemuir, while the Highland league side Turriff United drew 1-1 against Morton.
Hibernian host the Scottish Cup holders Hearts on Sunday in the Edinburgh derby at Easter Road and the top two teams in the Third division meet at Ibrox in the fourth-round tie between Rangers and Elgin.