Germany midfielder Michael Ballack volleyed Chelsea into the Champions League quarter-finals in a stirring 2-1 comeback victory at Stamford Bridge on Tuesday.

With the knockout round tie deadlocked at 1-1 after the first leg two weeks ago, Porto soaked up all the early pressure and took a surprise 15th minute lead through striker Ricardo Quaresma.

Chelsea equalised through Dutch winger Arjen Robben three minutes into the second half and Ballack smashed in a 79th-minute winner to secure a 3-2 aggregate victory for the English champions.

It was a fully deserved win for Chelsea and their manager Jose Mourinho, who had coached Porto to the Champions League trophy in 2004 before moving to Stamford Bridge.

The hosts had the lion's share of the possession and looked the more determined to win, though Porto remained a dangerous opponent throughout.

The Portuguese scored with their first real attack, Quaresma finishing off a neat four-man move by collecting Lucho Gonzalez's pass and running through to squeeze a low shot beneath the advancing goalkeeper Petr Cech.

It was the first goal Chelsea had conceded at home in this season's Champions League after three clean sheets in the group phase. 

CHELSEA CHANCES

They could easily have been level a few minutes later but Ukraine striker Andriy Shevchenko, who had equalised in the first leg, completely mis-kicked when Ashley Cole put over an inviting cross from the left.

Frank Lampard, captain in place of John Terry after his League Cup final concussion, fared little better on the half-hour mark.

Shevchenko broke into the area and unselfishly squared the ball into Lampard's path, only for the England midfielder to produce a tame shot straight at keeper Helton.

Robben was then booked for a clear dive in the penalty area as Chelsea tried to exert maximum pressure on Porto's well-drilled defence.

But the Portuguese, comfortable on the ball and snapping into tackles whenever Chelsea had possession, were able to cope with Robben's runs and the aerial threat posed by striker Didier Drogba.

Mourinho swapped defensive midfielder Claude Makelele for the more attack-minded John Obi Mikel for the second half -- which could not have got off to a better start for the hosts, thanks to some poor goalkeeping.

Just three minutes in, Robben dodged a tackle nearly 25 metres out and tried a speculative shot which Helton saw coming but somehow still failed to stop. 

Drogba then ballooned a shot into the stand before having another effort charged down in a packed area before Chelsea finally got their merited second goal.

Cole looped a high ball into the area from the left, Drogba headed it across to Shevchenko and the Ukraine striker nodded it hopefully towards to Ballack, who managed swivel round and volley past Helton.

The strike ensured that Chelsea, who beat Arsenal in a stormy League Cup final on February 25, stayed on course for more silverware in Europe.