Caley Thistle slipped to a 2-1 defeat at Morton, which lifted the Greenock club off the foot of the Championship into seventh spot and to within two points of ICT with a game to spare.
The result was a second loss in three games for ICT with their 4-1 win over Queen’s Park on Saturday coming on the back of a 2-1 home defeat to Raith Rovers.
ICT had the chance to move fifth had they drawn or won at Cappielow, but it means there is just five points between Airdrie in fifth and new basement team Arbroath, who face second-top Raith Rovers on Wednesday night.
Ex-Caley Jags forward George Oakley cashed in on a defensive mistake to put Morton in front and Jack Baird pounced to secure the points with 10 minutes to go.
Adam Brooks’ stoppage-time header came too late to salvage a last-gasp ICT point.
This was ICT manager Duncan Ferguson’s 10th match in charge since replacing Billy Dodds at the end of September. He watched from the stand as a result of his red card at Hampden on Saturday.
Four wins, three draws and now two defeats in the league along with a Scottish Cup victory over Cowdenbeath represents a strong response from the team which has the top-four as a genuine target.
That said, Ferguson has stressed his first target was to keep the club in the second-tier.
Morton’s 2-1 win at Arbroath at the weekend meant a victory here for the hosts would slice the gap between the two to a mere two points.
With three games within seven days, Ferguson made just two changes from the weekend.
In came Jake Davidson for Wallace Duffy, who had a calf injury, and Luis Longstaff took the place of benched Nathan Shaw.
Pre-match, Dougie Imrie had expressed concern that last season’s strong home form had deserted them, with just one win and five defeats prior to this clash the worst form in the division.
Many eyes were on in-form Caley Jags attacker David Wotherspoon, as the club await to hear whether the “strong” deal offer to keep him beyond his January contract will be successful.
And he had the first chance of note as he stepped in from the left and rifled a shot over Ryan Mullen’s crossbar.
Morton then tested Mark Ridgers as a 25-yard Robbie Muirhead free-kick, won by Lewis Strapp, was gratefully gathered by the ICT goalkeeper.
The Ton took the lead on 19 minutes and it came from a mix-up at the back as a slack Nikola Ujdur header allowed Oakley in and he hooked the ball over Ridgers. He celebrated with a smile in front of the visiting fans.
Inverness sought to respond and a fine pass from Billy Mckay picked out full-back Davidson, who was halted by a timely Strapp challenge as he got set to shoot on 33 minutes.
Twice Wotherspoon had low efforts flash past the goalposts, but Morton were going toe-to-toe overall in a no-holds-barred contest.
Another former Inverness player came close as Kirk Broadfoot drew a save from Ridgers after linking with Iain Wilson in the box.
Then, just before the interval, Billy Mckay beat the offside trap when Cammy Harper sent him clear, but Mullen touched the ball over the bar.
Ridgers made another telling stop early in the second half when he got down smartly to hold a stinging shot from Cameron Blues.
Morton kept the heat on and Oakley squandered a chance when he struck the ball wide after meeting a long throw-in from Strapp.
The Greenock side were enjoying the bulk of the play and pressure, sensing a second goal would likely end this contest.
Oakley continued to be a menace, winning a free-kick from Morgan Boyes, using his experience to draw a foul. Inverness defended the set-piece.
Wotherspoon, from a free-kick earned by Max Anderson, went for the target from out wide right, but Mullen was equal to it with 20 minutes left.
It was game over on 80 minutes when, from another long throw from Strapp, Baird was ideally placed to crash the ball past Ridgers.
Sub Brooks was on hand to crash home a close-range header in injury-time, but time ran out with ICT going for a leveller.
Inverness return to the Caledonian Stadium this Saturday as they take on Arbroath, while Ton tackle Queen’s Park at Cappielow.
MORTON (4-1-2-1-2): Mullen 7, French 6, Baird 6, Broadfoot 6, Strapp 8, Gillespie 6 (Power 57), Blues 6 (McGrattan 58)., Wilson 6, Crawford 6, Oakley 7 (Boyd 72), Muirhead 6.
Subs not used: Murdoch (GK), O’Connor, Bearne, O’Boy.
CALEY THISTLE (4-4-1-1): Ridgers 7, Davidson 6, Ujdur 6 (Lodovica 84), Devine 6, Boyes 6, Longstaff 6 (Shaw 76), Anderson 6, Gilmour 6 (Welsh 76), Harper 6, Wotherspoon 7 (Brooks 86), Mckay 7 (Sheridan 76).
Subs not used: Cammy MacKay (GK), Delaney, MacGregor, Thompson.
Referee: Lloyd Wilson.
Star Man: Lewis Strapp.
Attendance: 1,315.
