Three minutes into stoppage time Noah Sadiki hurdled a set of advertising hoardings, sprinted across a running track and hurled himself towards the ecstatic Sunderland fans.

Just as those supporters’ long-awaited party in Portugal had looked in peril of falling a little flat, the Democratic Republic of the Congo midfielder capitalised on excellent work from Enzo Le Fée and Granit Xhaka to half-volley an eye-catching first goal for Régis Le Bris’s side.

It ensured a largely unconvincing Sunderland came from behind to secure a second straight Europa League win and their relief was evident as every outfield player followed Sadiki in vaulting those hoardings to join the celebrations.

“It was a difficult game,” said Le Bris, Sunderland’s manager, with considerable understatement. “Torreense are a good team.”

With their home ground too small to accommodate European football, Leiria’s stadium stood in. It proved quite some substitute. An arena overlooked by a 12th-century castle constructed to defend this attractive medieval town against reconquest by the Moors created an imposing backdrop to Sunderland’s first competitive trip to the continent for 53 years.

Back in 1973 they were second division FA Cup holders. Torreense know the feeling. Having provoked widespread shock by beating Sporting to win the Portuguese Cup last spring, Luís Tralhão’s second-tier side relish their underdog status.

After winning at Lillestrøm last month, Torreense aimed to keep the surprises coming and duly left a heavily rotated Sunderland trailing at the break. Le Bris had left, among others, Xhaka, Le Fée and Nordi Mukiele on the bench ahead of Sunday’s vital Premier League trip to Bournemouth. This reshuffle permitted Luke O’Nien, a survivor from League One days, to captain Sunderland at a ground transformed into a sea of red and white. Although only 4,000 official tickets were allocated to the visitors more were readily available locally and estimates suggested the formal allocation had doubled.

As the sun dropped and the temperature fell from a high of 28C those fans watched a disjointed first half on Sunderland’s part. It seemed telling that the normally fairly impassive Le Bris looked distinctly, and unusually, irritated and frustrated at times.

Granted his humour was hardly helped by the referee’s penchant for detecting phantom Sunderland fouls but ominously Malick Fofana, a £30m summer signing from Lyon, struggled wide on the left. Ditto Nilson Angulo on the right wing.

Torreense, 15th in Portugal’s second tier might not have been threatening Robin Roefs’s goal but they succeeded in blocking and clearing pretty much everything that came their way. Indeed Tralhão’s defence looked almost as well fortified as that ancient castle towering over them.

Admittedly, midway through the first half Timur Tutierov, Sunderland’s centre-forward, thought he had headed the opener but that effort was disallowed due to shirt pulling on Omar Alderete’s part.

Then, with the interval fast approaching, Le Bris adopted hand-on-hips mode as Torreense reminded everyone precisely why they have built such a giantkilling reputation by scoring with their first shot.

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It came from centre-back Volnei Feltes. With Sunderland losing concentration and standing off, he was permitted to advance unopposed before cutting in on his right foot and sending a sublime 20-yard curler into the top corner.

Although Angulo forced Lucas Paes into a fine save he, like Fofana, was soon replaced as Le Bris made a quadruple substitution involving the introduction of Le Fée, Xhaka, Mukiele and Reinildo.

This infusion of quality left Torreense a little groggy and the midfielder Jules Ahoka –a highly impressive 20-year-old £8.5m summer buy from Royal Antwerp – was able to equalise by heading his first goal for Sunderland courtesy of Mukiele’s deft dink.

When the lively Tutierov went down in the box, the referee immediately pointed to the spot. However, a linesman’s flag indicated an offside so it seemed there would be no penalty. Or at least not until the video assistant referee intervened – lengthily; the eventual conclusion was no offside, no foul and no penalty.

No matter; when Le Fée dinked a gorgeous ball into the box and Xhaka played it into Sadiki’s path, the scorer waited for it to drop across him before applying the expert finishing touch on the half-volley.