Naomi Thomas opened the scoring in the 18th minute when finishing off well and despite creating chances Wales had to wait until injury time before adding to their tally when Berry scored her first try of the afternoon to give Non Evans an easy conversion and Wales a 12-0 halftime lead.
Wales burst out of the blocks in the second half with Evans finishing off in devastating fashion after Thomas had raced 70m from deep inside her own half to send the veteran over.
Wales lacked a clinical edge in the second spell and had to wait until the 72nd minute before scoring their next try. Prop Catrin Edwards pounced to score Wales's final try but in the dying seconds Italy had the final say when flanker Michela Este crossed the white wash to give fly half Veronica Schiavon an easy conversion.
Both teams had players sent to the sin bin with Wales losing the impressive front rower Rhian Bowden in the second half for 10 minutes after Italy had earlier lost Este on the stroke of halftime.
After recording their third win of the campaign, Lewis said: "We are pleased to post another win in the championship and book our ticket to the next World Cup. Italy were very confrontational in the first half and we didn't help ourselves in the contact area where the referee was particularly vigilant.
"Our focus is now on the game against Ireland next week. If we beat Ireland we will take the Triple Crown and second in the championship."
Scorers: Wales: Tries - Naomi Thomas, Non Evans. Mellissa Berry (2), Catrin Edwards; Cons - Non Evans (2)
Italy: Try - Michela Este; Con - Veronica Schiavon