Wales could still have a European finalist as Caerphilly produced a fine second-half performance to come away from Italy with an 18-point lead to take to the second leg at Virginia Park in two weeks time.
The game began well for Padova. Samoan No. 8 Nic Fitisemanu scored the opening try, and five points from the boot of Argentinean fullback Facundo Soler, gave the home side a fine start, but three Justin Thomas penalty goals kept the visitors in the match at the break, trailing by just one point at 10-9.
And it was a much improved Caerphilly side who came out in the second half, the Castletown scored three unanswered tries to break the Italian resistance.
The Caerphilly tries came from veteran winger Geraint Lewis - dubbed the 'Grey Shark' by coach Mark Ring - replacement No 8 Andrew Williams and Thomas. Thomas added two conversions to help establish a healthy lead.
HT: 10-9 Attendance: 1,000
Petrarca Padova: Try - N Fitisemanu; Pen - F Soler; Con - F Soler.
Caerphilly: Tries - G Lewis, A Williams, J Thomas; Pens - J Thomas 3; Cons - J Thomas 2.
Petrarca Padova: F Soler; F Faggiotto, M Bergamasco, R Salvan, M Baroni; A Marcato, P Travagli; A Muraro, L Ferraro, M Matteralia, D Tumiati, G Llanos, M Bortolami, H Mazzocco, N Fitisemanu
Replacements; G Boccalon, L Piovan, R Bortolato, R Rampazzo, D Dalla Nora, M Farsura, A Chillon
Caerphilly: S Tuipulotu; T Taufahema, R Boobyer, R Higgitt, G Lewis; J Thomas, A Chilten; L Manning, F Vunipola, D Sweet, D Davies, P Jones, J El-Abd (capt), S Grainger, M Workman
Replacements; F McLoughlin, R Howells, T Carless, G Liddon, N Rouse, G Jones, A Williams
Referee: Gregg Davies (Scotland)