Second behind Caisse D'Epargne rider Erviti in the 175.7km hilly stage from Tarragona to Vilanova i la Geltru was Belgian Romain Zingle, with Belgium's Greg Van Avermaet third.
Rodriguez claimed the leader's jersey from Spaniard Igor Anton after snatching a two-second time bonus early in the stage.
Pre-race favourite Andy Schleck of Luxembourg failed to start the stage after his Saxo Bank team withdrew him from the race for breaking internal team regulations.
The Tour of Spain finishes in Madrid on September 19.
STAGE RESULTS
1. Imanol Erviti (Spain / Caisse d'Epargne) 4hrs 13mins 31secs
2. Romain Zingle (Belgium / Cofidis) +37"
3. Greg Van Avermaet (Belgium / Omega Pharma - Lotto)
4. Mauro Finetto (Italy / Liquigas)
5. Javier Moreno (Spain / Andalucia)
6. Anders Lund (Denmark / Saxo Bank)
7. Christophe Le Mevel (France / FDJ)
8. Gianpaolo Cheula (Italy / Footon)
OVERALL STANDINGS
1. Joaquim Rodriguez (Spain / Katusha) 42 hrs 11 mins 49s
2. Igor Anton (Spain / Euskaltel) +2"
3. Vincenzo Nibali (Italy / Liquigas) +4"
4. Xavier Tondo (Spain / Cervelo) +44"
5. Jean Christophe Peraud (France/Omega Pharma-Lotto) +54"
6. Ruben Plaza (Spain / Caisse d'Epargne) +1:17"
7. Ezequiel Mosquera (Spain / Xacobeo) +1:20"
8. Nicolas Roche (Ireland / AG2R) +1:21"