Manchester - Manchester United are in danger of losing their grip on the Champions League after the holders were held to a 2-2 draw by Porto in Tuesday's quarter-final first leg.
Sir Alex Ferguson's side looked to have earned a barely deserved victory when Carlos Tevez stabbed United into the lead with five minutes to play at Old Trafford.
But Porto substitute Mariano equalised four minutes later to give the visitors just reward for an impressive display and leave United almost certainly needing to become the first English side to win at Porto's stadium next week.
Jesualdo Ferreira's side had gone ahead early in the first half through Cristian Rodriguez and could have been further in front before a calamitous mistake from Bruno Alves set up Wayne Rooney's equaliser.
Not for the first time in recent weeks, United looked nervous in defence and short of urgency in attack. Now they face a testing trip to the Dragao Stadium next Wednesday knowing only a victory or an extremely high-scoring draw will keep alive their hopes of retaining the Champions League trophy.
After staging a dramatic late fightback to beat Aston Villa on Sunday, Ferguson had expected the momentum from Federico Macheda's winner to give his players enough adrenaline to ignore their aching limbs as they returned to action 48 hours later.
He couldn't have been more wrong. Although they enjoying a record-breaking run of cleansheets earlier this season, United's defence has sprung a few leaks in recent weeks.
They looked no more secure in the opening minutes. Lisandro Lopez, Porto's Argentine striker, almost took advantage as he sprinted past Jonny Evans and forced Edwin van der Sar into a sprawling save.
Porto attacked United's soft centre again in the fourth minute and this time they were able to punish another sloppy piece of defending.
When Cristiano Ronaldo lost the ball, Lucho Gonzalez crossed towards Rodriguez, but Evans should have cleared easily. Instead he scuffed only as far as Rodriguez, who drove a fine low strike past van der Sar into the far corner.
United had been far too careless in possession but they sparked into life when Ronaldo met John O'Shea's cross with a header that Helton pushed away for a corner.
That hardly amounted to a ceaseless assault however, which made Alves's spectacular implosion in the 15th minute so hard to understand.
Under no pressure at all, the Porto defender passed back to Helton without looking. Alves hadn't noticed Rooney lurking behind him and the United striker gratefully seized the gift before clipping a deft finish past Helton.
Despite that lifeline, there was still no sign that United had found their rhythm.
Porto could have had a second goal if Rodriguez had steered his close-range header either side of van der Sar instead of straight at the Dutch keeper.
While United laboured, Porto looked impressive whenever they went forward and van der Sar was at full stretch to turn over a long-range shot from Raul Meireles.
After watching such a lackluster effort from his players, Ferguson must have been tempted to give them the full 'hairdryer' treatment at half-time.
United responded at last and Rooney produced a superb chip from 30 yards that Helton just managed to tip over. From the resulting corner Vidic headed goalwards but Helton made a brilliant stop to deny the Serbian.
The Portuguese champions kept their nerve and Lisandro's skimming strike needed an alert stop from van der Sar before Aly Cissokho tested the United keeper from long-range.
This was no way for United to celebrate their 100th home game in the European Cup and Ferguson made his last two changes, opting to leave Italian sensation Marcheda on the bench and send on Tevez and Gary Neville instead.
Tevez made an instant impact. In the 85th minute Rooney flicked on Neville's throw and Tevez showed great determination to turn the ball home from close-range.
But Porto refused to accept defeat and staged the kind of dramatic finale that United have made their trademark.
Lisandro swept over an 89th minute cross that O'Shea could only help on to Mariano and the Porto substitute stabbed his shot over van der Sar. It was no more than Porto deserved.