Noah Sadiki's 93rd-minute winner helped Sunderland continue their perfect record in the Europa League with a come-from-behind victory at Torreense.
Torreense, from Portugal's second tier, took a shock lead with their only shot of the first half when centre-back Volnei Feltes drove forward from the halfway line before hammered a stunning strike into the top corner.
Sunderland levelled in the 67th minute through summer signing Jules Ahoka, who scored his first goal for the club by heading in Nordi Mukiele's cross.
The Black Cats were given an 88th-minute penalty for a clash between Mohamed Ali Diadie and Timur Tutierov but Albanian referee Enea Jorgji overturned his initial decision after looking at the pitchside monitor.
But the visitors still managed to have the final say when Sadiki also scored his first goal for the club with a thunderous strike into the corner in the 93rd minute.
Sunderland were playing their first away match in a European campaign in 53 years, having beaten AZ Alkmaar 1-0 at home in their Europa League opener last month.
With visits to AC Milan and Aderlecht to come, this was supposed to be Sunderland's easiest fixture on the road.
Torreense have not competed in the top tier of Portuguese football since 1991-92 and qualified for this season's Europa League by shocking Sporting Lisbon in the Portuguese Cup final.
Thursday's match was being played 47 miles from Torreense's home ground which does not pass Uefa's stadium criteria, while manager Luis Tralhao was not able to do post-match media duties as he does not hold a Uefa Pro license.
But, rather than cruising to victory as many would have predicted, Sunderland, who made eight changes, had a nervy night - unable to break their opponents down despite their hatful of chances.
They had an opener ruled out for a push by Omar Alderete just before Tutierov turned the ball into the net following a long throw.
Home keeper Lucas Paes also had a fantastic game making fine reflex saves to keep out Nilson Angulo and Ahoka.
Sadiki's winner means Sunderland, who are just a point above the Premier League relegation zone after a stuttering start, recorded their first away victory of the season.
After the match, Sunderland manager Regis le Bris said the result was proof his side are "growing up and need experiences".
"We're back in Europe 53 years later and winning this game right at the end is very special. It is a symbol, and important, I think for many reasons," he said.
"I'm enjoying every single second. It's going to be challenging and demanding but exciting as well. We want to embrace the challenge."