Chelsea 2 Porto 1 (3-2):Michael Ballack earned Chelsea a place in the quarter-finals of the Champions League with a late winner at Stamford Bridge, as Jose Mourinho's former club threatened to dump the Londoners out of the competition after Ricardo Quaresma had given them a precious half-time lead.

Arjen Robben restored parity three minutes after the break and Ballack's strike — his first since November — put Chelsea in the last eight.

It had looked distinctly worrying for the Blues in the opening half when they were often left bamboozled by some clever one-touch football by Porto.

Chelsea enjoyed the lion's share of the possession but, largely due to some competent defending from the visitors, they failed to put any real pressure on Porto goalkeeper Helton.

Chelsea had started promisingly with Andriy Shevchenko attempting to beat Helton with a curling effort from 25 yards in the opening minute.

But the threat from Porto's quickfire football materialised with deadly effect in the 15th minute when, for once, the normally unflappable Michael Essien was caught on the hop.

Essien has been a rock at the heart of Chelsea's defence in the absence of captain John Terry but he was hopelessly out of position as Lucho Gonzalez fed the ball through to Quaresma.

The Porto winger showed why his reputation is growing throughout Europe by leaving Carvalho in his wake as he drew goalkeeper Petr Cech out of his goal before slipping the ball under his body and into the net.

Chelsea huffed and puffed away but too many of their more reliable stalwarts were having an off-day.

Claude Makelele, Ballack and Frank Lampard were strangely off the pace for much of the half and the sense of growing frustration was heightened further when Robben was booked for diving in the 34th minute.

Referee Roberto Rosetti was correct to show the Dutchman the yellow card after he tumbled theatrically to the ground when attempting to round Fucile.

Chelsea's frustration further erupted moments before the interval when Drogba was accused of elbowing Fucile. The Porto defender then incurred the wrath of the home fans by falling to the ground when asked to leave the field for treatment to a cut head.

It was all too much for Mourinho, who stormed down the tunnel to the dressing room minutes before the half-time whistle. Makelele, clearly not enjoying his best spell at Chelsea, was understandably replaced by Mikel Jon Obi at half-time.

It was a different Chelsea in the second half as they gave Porto little time on the ball.

The extra work ethic was paying dividends with Robben causing panic in the Porto defence every time he was presented with the ball.

Helton was clearly rattled by his mistake and he was lucky to survive on two other occasions when he failed to deal with crosses adequately enough.

The second goal duly arrived in some style 11 minutes from time as Chelsea carved open the Porto defence with ease.

Ashley Cole's cross picked out Drogba and for once the Ivorian used his power to good effect to steer the ball to the unmarked Shevchenko.

The Ukrainian captain unselfishly opted to head the ball back into the six-yard box where the waiting Ballack despatched a low volley into the corner of the net.

It was the signal for party time at Stamford Bridge and Chelsea, knowing one more goal from Porto would put them out on away goals, kept their composure to keep them on course for an unprecedented quadruple.