For 33 minutes, Chelsea FC's UEFA Champions League dream was in peril but second-half goals from Arjen Robben and Michael Ballack sent Jose Mourinho's side into the quarter-finals for the third time in four years and at the expense of his former club.
When Ricardo Quaresma scored early on for FC Porto, it was only the second game in nine home matches in the competition that Chelsea had conceded. They are used to having things their own way here, but Robben got them back on track immediately after the interval and Ballack played the trump card with a close-range volley with eleven minutes remaining.
Andriy Shevchenko sent an angled drive fizzing over the top with barely a minute gone, then Ballack stooped to apply a header to Frank Lampard's corner. Despite the encouraging start, Chelsea were not at their vintage best, with a number of passes going astray. Porto, meanwhile, demonstrated that they could make significant inroads into the Chelsea half.
Even so, it was still a surprise when the visitors broke through to score a stunning goal and shatter Chelsea's proud defensive record at Stamford Bridge in this competition. Their counterattack began with Pepe who found Lisandro López, and when Lucho González joined in the midfielder laid a superb pass into the path of Quaresma, who had broken clear of the blue shirts with a straight run on goal. With only Petr Čech to beat, the winger remained composed and finished coolly beneath the goalkeeper.
This was potentially a far more serious blow to Chelsea than Porto's opening strike two weeks ago, as the Portuguese title-holders now had a crucial away goal. The Premiership champions poured forward in response, with Ashley Cole taking every opportunity to surge deep into Porto territory. From one cross Shevchenko missed his kick and Didier Drogba scooped the ball over. From another, Helton got his decision-making all wrong but Ballack was unable to take advantage. Lassana Diarra charged forward down the other flank and when he was impeded, the stage was set for Robben to curl in a teasing centre. Unfortunately for the hosts, his delivery flew over the heads of both Shevchenko and Ricardo Carvalho.
At the interval Mourinho took off Claude Makelele in favour of John Obi Mikel. The young substitute had been on the field for only three minutes before he was joining in the celebrations that followed Robben's timely equaliser. The winger's low shot should have demanded no more than a routine stop from Helton but the goalkeeper allowed the ball to spin off his body and drop over the line.
It was just what Chelsea needed before any serious doubts began to infect their play, although Porto had not come close to threatening a second goal as they were forced to soak up a lot of pressure. They became careless in possession and the tide was definitely with Chelsea, though their rhythm was not helped as first Shevchenko then Ballack required treatment. The pair recovered, though, to combine for Ballack to score the late winner and take the Londoners into Friday's draw.