Porto grabbed an 89th-minute equaliser to secure a 2-2 draw against Manchester United at Old Trafford in the first leg of their Champions League quarter-final tie.
Cristian Rodriguez capitalised on some nervy early play from the home side to give Porto a lead in only the fourth minute.
The excellent Wayne Rooney then provided an equaliser after a quarter of an hour before he appeared to have won the game when he flicked the ball into the path of Carlos Tevez to put them 2-1 up with just four minutes to play.
Yet Porto silenced the home fans in the dying moments when Mariano Gonzalez was left in acres of space to level the scores and give his side a potentially crucial advantage on away goals.
When the draw was made, United could hardly have been happier: avoiding Barcelona and the three other English sides was always the hope, and Porto were ranked by almost every bookmaker as the tournament’s rank outsiders.
Nonetheless, Jose Mourinho’s Porto side had stung United in 2004 and, with the current team having dispatched Atletico Madrid in the previous round, Sir Alex Ferguson was at pains to stress that the game was not a formality.
"It would be stupid to think this is going to be easy," he said ahead of the game.
"Complacency does not come into it as far as we are concerned. We will be going at it full throttle in order to get the result we want."
In the event, United struggled to get out of first gear in an error-strewn start. As Ferguson admitted at half-time, they looked not so much complacent as nervous, and they paid a heavy price.
Brazilian striker Hulk highlighted the threat in the opening moments as he broke free before stinging Edwin van der Sar’s fingertips with a fierce long-range effort.
United failed to heed the warning and, just four minutes in, Cristiano Ronaldo lost the ball, Jonny Evans failed to deal with the ensuing cross and Cristian Rodriguez picked the ball up on the edge of the area before drilling into the bottom corner.
Ferguson may not have expected an easy game, but he surely could not have anticipated another such disjointed defensive display after the return of Nemanja Vidic.
Porto dominated throughout the half, and it was only thanks to a colossal error from Bruno Alves that they were able to level the scores: in the 15th minute, the Portugal defender played a backpass blindly to the alert Rooney, who took the chance with aplomb.
It was a quite unbelievable gift at this level, but it was not until the final minutes of the half that there were any signs of United bouncing back from that early blow, and Ferguson had reason to be grateful that the score was still 1-1 at half-time.
In the second half, the home side still looked capable of unforced errors, but passes were generally finding their target and they were creating space in the midfield.
As they grew in confidence, Rooney came close to providing an effort that encapsulated the mood. On 59 minutes, the England striker attempted an audacious chip from outside the area, forcing Helton to tip over at full stretch. From the corner that followed, Vidic forced a good save with a powerful header.
Porto continued to attack on the break, though, and substitute Tevez very nearly gave away a penalty just moments after his 72nd-minute arrival when he appeared to bring down Hulk in the area, but the referee waved away the appeals.
But United continued to have the better of it, and another substitute, Gary Neville, put in a dangerous cross that just eluded Rooney with the goalmouth gaping.
It was not until five minutes from the end that they made their new-found superiority count. Rooney did superbly well to flick on a throw-in into the area, and Tevez was swift to react, smashing the ball into the roof of the net.
However, Porto were to have the final say as Lucho Gonzalez crossed into the area to find Mariano, who was left to provide a relatively simple finish in acres of space.
United had been here before, with Mourinho’s Porto grabbing a crucial away goal in the dying seconds at Old Trafford five years previously.
Unlike the Inter boss, Jesualdo Ferreira did not race away down the touchline, but this was still a significant blow that could have serious psychological repercussions as Ferguson looks to steer his side back on track.
STAR MAN: Wayne Rooney (8). The England striker's distribution was occasionally wayward, but he was at the heart of all his side's best work, scoring one, creating another and always looking to make things happen. Click here for full Opta match stats and player ratings.