Di Luca, who also won at the hilltop sanctuary near Naples in 2001, edged out fellow general classification contenders Riccardo Ricco and Damiano Cunego in a sprint at the summit of the 17km climb.
Di Luca - whose team won the team time trial stage on Saturday - continues to gain time on his major rivals.
Of the overall favourites Italian Gilberto Simoni was 15th and lost 15 seconds while former champion Paolo Savoldelli of Italy and Ukraine's Yaroslav Popovych both dropped 34 seconds.
Liquigas savour success
The riders were back in the saddle for the 153km stage from Salerno after Tuesday's rest day with Markel Irizar (Euskaltel), Yuriy Krivtsov (Lampre) and Pavel Brutt (Tinkoff) setting off on an early escape.
They built up a healthy lead after crossing the third category climb Picco S. Angelo which extended to seven minutes after a mass fall in the peloton following a downpour.
World champion Paolo Bettini was one of the worst affected and had to receive treatment from the race doctor on a rib injury. He eventually laboured home five minutes behind the winner.
The trio of escapees were eventually caught with around 8km to go thanks to the hard work of Saunier Duval and then Liquigas.
Diminutive Mexican climber Julio Alberto Perez Cuapio counter-attacked and held a 24 second lead on the ascent before the green train led by England's Charly Wegelius and then Vincenzo Nibali and Franco Pellizotti saw him caught with a kilometre to go.
Di Luca peeled off Pellizotti's wheels inside the final 400m and held off a determined challenge from Saunier Duval's rising star Ricco to take his fifth stage victory in the race.
"It was a great stage for us," Di Luca said.
"I have to thank my team mates because I only had do the last 200 metres. They rode perfectly and protected me until I sprinted to the line. This wins is all thanks to them."
"I think this is the strongest I've ever been in my career but it's hard work leading the Giro and if we tried to defend the pink jersey my team mates would be tired and I'd be on my own in the mountains when I need them most."
The 31-year-old now leads by 25 seconds overall from Pellizotti with Cunego sixth at 54 seconds, Savoldelli eight at 1:07, Popovych 19th at 1:43 and Simoni 23rd, two minutes behind.
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