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Chelsea came from behind to beat Porto 2-1 and book their place in the quarter-finals of the UEFA Champions League.

The Blues were firm favourites to progress following a 1-1 draw in Portugal but they were almost punished for a dismal first half performance as Ricardo Quaresma struck in the 15th minute.

However, the game turned on a goalkeeping gaffe from Helton after 48 minutes which gave Arjen Robben Chelsea's equaliser, and Michael Ballack popped up 11 minutes from time to fire the Premiership champions through to the last eight.

Andrei Shevchenko almost opened the scoring inside the opening minute when he drilled a powerful shot just over the crossbar from 20 yards, but his effort failed to inspire his team-mates as Chelsea started the game sluggishly.

Jesualdo Ferreira opted to take a defensive approach to the game and left striker Helder Postiga out of his starting line-up, but his plans were vindicated as Porto took the lead with a superb goal on the counter-attack.

Lisandro Lopez showed quick feet in midfield to get the ball to Lucho Gonzalez and he released Quaresma with a fine pass between Chelsea centre-backs Michael Essien and Ricardo Carvalho. Quaresma showed no nerves with just Petr Cech to beat and slotted his shot coolly past the keeper.

Chelsea were frustrated in their attempts to force a quick equaliser and did not manage to work Porto keeper Helton once in the remainder of the first half.

Didier Drogba looked a shadow of his usual self and, after clashing heads with Bruno Alves, spurned a good chance from inside the area as he shot high and wide.

Robben was presented with a yellow card rather than a penalty after an outrageous dive over Raul Moreiles, and as Chelsea's front six laboured in possession it was left to defenders Ashley Cole and Essien to provide any threat at all.

Cole's devilish cross from the left almost broke for Ballack after a poor clearance by Ricardo Costa, but the Porto defender made amends to poke the ball away from the German before he could shoot.

Porto looked much more assured in possession and their most worrying moment of the half came when Jorge Fucile was caught by the flailing arm of team-mate Paulo Assuncao and was left with blood streaming down his face.

Fucile was not seriously hurt and returned for the second half, while Jose Mourinho brought on John Obi Mikel for Claude Makelele.

It took just three minutes for Chelsea to get back into the game as a howler from Helton gifted Robben an equaliser. The Dutchman's low shot from 25 yards was completely lacking in venom but proved too hot for Helton to handle as the ball spilled over the goalkeeper's dive and into the net.

The goal lifted Chelsea's whole game and they were sharper in possession as they penned Porto back into their own half, but the Portuguese side remained defiant in defence.

Porto did well to prevent Chelsea from taking shots from distance, but Helton looked ill at ease at set pieces and only just succeeded in punching away a couple of free-kicks.

Chelsea's sustained pressure eventually told after 79 minutes as Shevchenko headed down Cole's cross and Ballack finished with a neat volley from eight yards.

At 2-1 down all was not lost for Porto, who knew that an equaliser would take them through on the away-goals rule.

They committed more men forward in pursuit of a goal and Bruno Moraes was introduced, but Chelsea held on comfortably in the last ten minutes to keep their dreams of European glory alive.

 Teams

Chelsea Cech, Carvalho, Cole, Ballack, Diarra (Ferreira 66), Essien, Lampard, Makelele (Mikel 45), Robben, Drogba, Shevchenko (Kalou 85)
Subs Not Used  Hilario, Boulahrouz, Bridge, Wright-Phillips
Booked  Robben, Diarra
Goals  Robben 48, Ballack 79

Porto Helton, Alves, Cech (Ibson 55), Costa, Fucile, Pepe, Assuncao, Gonzalez, Meireles (Adriano 55), Quaresma, Lopez (Moraes 82)
Subs Not Used Baia, Alan, Postiga, Paulo
Booked Quarsma, Fucile, Adriano
Goals  Quarsma 15

Attendance 39,041

Referee  Roberto Rosetti (Italy)