Assistant manager Ray Wilkins admitted Chelsea had failed to fire on all cylinders but overcame Cardiff City in the FA Cup due to their 'professionalism'.
The Blues - minus embattled skipper John Terry who has been given leave to tend to his personal problems - booked their place in the quarter-finals following a 4-1 victory over the Championship side.
In-form Ivory Coast striker Didier Drogba scored after just two minutes and provided the impetuous for a comfortable victory, with further goals from Michael Ballack, Daniel Sturridge and Salomon Kalou adding a gloss to the final scoreline.
"It's not so much their class as their professionalism that shines through," explained Wilkins at full-time.
"Some people might have thought it would be a cushy game for us but those guys took it by the throat and pulled us through when we weren't paying particularly well.
"We anticipated a hard game and obviously Cardiff from a lower division will work their rocks off basically and that's what they did.
"They played exceptionally well first half and gave us a lot of problems, but thankfully we managed to up the tempo in the second half and it became a tad easier for us."
Meanwhile, Wilkins has backed Joe Cole to rediscover his best form despite the England midfielder being substituted for Kalou on half-time.
He added: "When you've had a long (injury) layoff like Joe it is a shock to the system and it takes a while to get back to tip-top form and that will come with Joe no problem at all.
"He's working extremely hard day by day so we don't see any problems there at all."