Two penalties in quick succession either side of half time left Southampton on the receiving end of an Emirates FA Cup shock at St Mary’s.
Despite dominating the first period, Saints were stung from the spot by League Two Grimsby after Lyanco was penalised for handball following a lengthy VAR delay shortly before the interval.
Gavan Holohan, who kept his cool to send Alex McCarthy the wrong way, was gifted another chance from 12 yards when Duje Ćaleta-Car lashed out with a stray arm that caught Danilo Orsi five minutes into the second period.
When Holohan scored once more, Saints were desperate. Ćaleta-Car did pull one back, sparking total domination in the remaining 25 minutes, but despite Saints’ best efforts, the visitors held on for an historic victory that sends them into the quarter-finals.
Rubén Sellés stayed true to his word to rotate in numbers, making nine changes to his team that took to the field at Leeds last time out.
Romain Perraud and Roméo Lavia were the sole survivors, meaning Kyle Walker-Peters captained the side with James Ward-Prowse among the substitutes.
Saints were straight on the front foot, winning an early corner even before Lavia’s pass outfoxed the Grimsby defence, allowing Sékou Mara to escape and go eye to eye with Max Crocombe, who stood tall and blocked the Frenchman’s shot, which was struck with power but too close to the goalkeeper.
Grimsby, a non-league outfit last season, could not get out of their half for the first ten minutes, but then began to grow in confidence.
Right-back Michee Efete went to ground in the penalty area, prompting loud appeals, as a sustained spell of pressure sent the volume of an already boisterous away end up a few more notches.
But Saints had the talent to raise the level on the pitch. Walker-Peters slid a low pass in behind the defence for Moussa Djenepo, whose cutback was tucked away nicely by Mara inside Crocombe’s near post, but Djenepo was flagged offside in the build-up.
Saints’ brightest spark was Lavia. Midway through the first half it was him winning the ball on the halfway line to initiate another attack, as Carlos Alcaraz drove forward and fed Mislav Oršić to his left, but the Croatian could not get his shot to curl enough to find the bottom corner at the end of another promising break.
Another Alcaraz surge saw him look left again before continuing his run into the box, this time to meet Adam Armstrong’s cross, but the Argentinian stooped to head it when the ball was too low.
Grimsby were always going to have to pick their moments to attack, and one duly arrived four minutes before half time when Joshua Emmanuel broke free down the right wing.
When Emmanuel’s cross took a slight deflection on the way through, it fell nicely for Harry Clifton to take the ball on his chest, but McCarthy advanced to thwart the winger at close quarters.
Just as Saints thought they’d survived a scare, a lengthy VAR check ensued, as it transpired Emmanuel’s cross had brushed Lyanco’s outstretched arm in the box.
Nobody inside the stadium had spotted it, but referee Thomas Bramall was sent across to consult the pitchside monitor, which usually only means one thing.
After a break in play totalling four minutes, Holohan took a deep breath and sent McCarthy the wrong way to send more than 4,000 travelling fans from Cleethorpes into raptures at the opposite end of St Mary’s.
Still Saints should have gone in at half time on level terms, but Mara could only steer the ball over the bar from six yards after a low corner somehow evaded everybody in front of him.
Sellés resisted the urge to make substitutions at half time, but initially saw a reaction from his players when Djenepo danced his way into a shooting position, only to see his low drive deflect just wide.
But at the other end, Saints were their own worst enemy. An innocuous long ball forward sparked a tangle between Ćaleta-Car and Orsi, which clearly started outside the box.
As McCarthy claimed the ball, the tussle continued and Ćaleta-Car lashed out with an arm into Orsi’s back
leading the Grimsby man to hit the ground theatrically and Bramall to point to the spot for a second time.
With St Mary’s in a state of shock, Holohan stepped up once more, this time in front of the Grimsby supporters, to beat McCarthy high to his right.
Sellés did turn to his bench with the hour mark approaching, introducing Ward-Prowse and Samuel Edozie with Saints in need of rescuing.
It would take a Ward-Prowse set-piece to halve the deficit. After Djenepo reached the byline to win a corner down the left, the skipper crossed and Ćaleta-Car coolly volleyed home at the far post. Game on.
Theo Walcott and Joe Aribo, already warming up before the goal, were immediately introduced as Saints smelt blood.
The pair joined forces as a new-look front two, with Walcott denied by an important block before Aribo stretched for Lyanco’s header back across goal, but could not quite connect from point-blank range when any touch would do.
The visitors’ chance to reclaim their two-goal lead was spurned when John McAtee’s touch was heavy, allowing McCarthy to save bravely at his feet and spare the blushes of Perraud, whose touch had escaped him.
Then Walcott thought he’d equalised with ten minutes left, sneaking in behind the Mariners' defence to finish off a floated Ward-Prowse free-kick, but once more VAR intervened, and the substitute was given offside.
There was a home debut for 17-year-old Dominic Ballard, introduced in the 85th minute, as Sellés threw on every attacker at his disposal.
Instead it was a defender, Walker-Peters, who went closest in six minutes of added time, but his shot whistled a whisker wide on a famous night for Grimsby at St Mary’s.