Steve Bruce admitted he saw Hull's European dreams slipping away under the KC Stadium floodlights before substitute Sone Aluko fired the Tigers into the Europa League play-offs.
The FA Cup runners-up needed to beat ambitious Slovakian club AS Trencin, against who they secured a goalless draw in the first leg, on Thursday night to progress from the qualifying third round.
But in front of a roaring crowd at the KC, Trencin took the lead through Tomas Malec inside two minutes to seriously endanger the Tigers' chances.
Ahmed Elmohamady headed an equaliser on the half-hour mark and, just when it looked like Trencin would succeed thanks to the away goals rule, Aluko climbed off the bench to prod home an 80th-minute winner.
A relieved Bruce will on Friday get to discover who his team face in the play-offs later this month, but for a long time he thought the club's maiden European adventure was over before it had even begun.
"You're always worried when there's 15 minutes to go and you're thinking about all the hype and euphoria, but you're about to fall at the first hurdle," he said.
"But we've stuck at it and, not for the first time, we've seen them do it.
"It's been difficult but we've got through it and let's enjoy the draw tomorrow."
Trencin had no intentions of rolling over, or sitting back once they had taken a shock lead, and strikers Shane Long and Yannick Sagbo both failed to provide sufficient bite in the quest for a comeback.
And Bruce has warned his players they need to start putting their chances away.
"I think that's the biggest lesson," he added. "Playing in the Premier League's the same, and when you play in Europe you don't get that many.
"To be fair we've missed three or four glorious chances and thought we weren't going to get it.
"We made an awful start and that put everyone on edge for 20 minutes, not just the players, but the crowd, who I think were fantastic.
"There was a frustration there but thankfully we've seen it through."
Elmohamady's close-range header brought Hull back into contention and it fell to Aluko, whose introduction off the bench followed that of home debutant Tom Ince, to complete the comeback.
Bruce added: "I honestly believe that the ability to bring people off the bench is what won us the game.
"When you can bring on people like Aluko and Ince, that gives you a freshness and changes the game.
"Of course, when one of them comes on and scores then that's great. I'm delighted about it."
While Bruce watched victory being snatched from the jaws of defeat, the experience of Trencin boss Martin Sevela was the exact opposite.
He said: "We are very sad that we didn't go through. We were 10 minutes from the end - and from victory.
"We started the game very well, scored the goal, had great chances and were dangerous.
"It took them a little while to get themselves together after we scored but they were physically very well prepared and I would like to congratulate Hull City for going further.
"I wish them all the best and I hope they get to the Europa League (group stages) and do well."