But that fantastic result against the pre-tournament favourites will mean nothing if they don't follow it up with another win at the Arms Park.
Harlequins managed a 13-13 draw at the Stoop in Round Two but with the Blues having home advantage, the Welsh region is expected to win.
Harlequins make seven changes to the team that started against Leicester last Sunday (January 6th) in the Guinness Premiership for the game against the Blues in the Heineken Cup at Cardiff Arms Park tomorrow.
In the pack, injuries force a number of changes including Jim Evans and Chris Robshaw who both picked up knocks against Leicester. As a result Tom Guest will replace Robshaw on the blindside flank and Nicolas Spanghero, who returns from injury himself, will start in the second row alongside James Percival, and Ollie Kohn starts on the bench. The front row will remain the same with the only other change being the return of captain Paul Volley in replace of Will Skinner.
In the backs, Tom Williams comes in on the wing for the injured Ugo Monye, who is another casualty from last Sunday's match, whilst Haldane Luscombe replaces Jordan Turner-Hall to partner De Wet Barry in the centres. Scrum half Steve So'oialo is promoted to the starting XV from the bench and Danny Care will take his spot amongst the replacements alongside academy centre Tosh Masson and Phil Davies.
With Duane Goodfield suffering a slight injury to his shoulder in training, Andy Powell suffering with flu and Tom James also not recovered from a throat and kidney infection, Cardiff Blues boss Daivid Young had only twenty-five fit players to choose from.
Speaking ahead of the match he said, "I think this is the first season we've had a realistic opportunity of reaching the last eight for a number of years. There's only one reason we haven't before and that's because we haven't been good enough."
"However, we've compiled a squad which is very competitive and I think we've shocked a few people that we are in this position. What we have to do is keep our squad fit for both games, as we are a bit stretched, and take our opportunities. It's not an easy path, we still have a lot to do, but we feel are capable of doing it," added Young.
Cardiff Blues: B Blair; J Roberts, T Shanklin, D Hewitt, G Thomas; N MacLeod, J Spice; G Jenkins, G Williams, T Filise, D Jones, P Tito, M Molitika, M Williams, X Rush (Capt)
Replacements: J Yapp, D Goodfield, S Morgan, M Lewis, R Rees, R Jones, J Robinson
Harlequins: M Brown; D Strettle, H Luscombe, D Barry, T Williams; C Malone, S So'oialo; C Jones, G Botha, M Ross, J Percival, N Spanghero, T Guest, P Volley (Capt), C Hala'Ufia
Replacements: T Fuga, R Nebbett, O Kohn, P Davies, D Care, A Jarvis, T Masson
Referee: Romain Poite (France)