MILAN, Jan 13 (Reuters) - AC Milan beat Napoli 5-2 on Sunday to record their first league win at the San Siro this term with Pato scoring on his debut and Ronaldo netting twice to open his season's account.Ronaldo, playing in only his second match this season after injury problems, gave Milan a 15th-minute lead when his shot ballooned off Napoli keeper Gennaro Iezzo and just crossed line before Andrea Cupi hooked clear. Napoli equalised when Ezequiel Lavezzi's cross was toe-poked in by Roberto Sosa, but Milan were soon back in front when Iezzo denied Brazilian 18-year-old Pato twice only for Clarence Seedorf to smash home. A clumsy foul by Kakha Kaladze on Lavezzi allowed Maurizio Domizzi to pull Napoli level from the penalty spot after 38 minutes of an enthralling first half. The second period belonged to Milan however, with a diving header from Ronaldo, Kaka's crisp shot and Pato's cool finish sealing the win in style. The European champions remain 12th but have three games in hand on most clubs having won the Club World Cup in Japan in December before Serie A's three-week midseason break. Pato, part of a three-pronged Brazilian attack, joined Milan from Internacional in the close season but was ineligible until now. He was nervous in the first half but gradually improved. 'I am not the phenomenon, Ronaldo is,' the much-heralded Pato told Sky. 'I told him that after the goal.' DEBATABLE PENALTY Earlier unbeaten Inter Milan maintained their seven-point lead at the top of Serie A after Zlatan Ibrahimovic's double helped the champions to a 3-2 win at a battling Siena. Ibrahimovic converted a debatable penalty on 26 minutes after Julio Cruz had collided with Paul Codrea. Siena equalised quickly when Inter defender Ivan Cordoba appeared to get the last touch under pressure from Massimo Maccarone but the champions went back in front on the stroke of halftime when Esteban Cambiasso slid in Ibrahimovic's pass. The Swede grabbed his 11th of the league campaign in the second half with a cracking shot and despite Fernando Forestieri's late goal, Inter hung on for their seventh straight Serie A win and 11th successive victory in all competitions. AS Roma moved back up to second but had to come from behind to beat Atalanta 2-1 thanks to Francesco Totti's low freekick and Mancini's thumping strike. Luciano Spalletti's side went above Juventus, who drew 1-1 at Catania on Saturday and are now three points behind in third, while Udinese stayed fourth after Fabio Quagliarella secured a 1-0 win at bottom side Cagliari. Fiorentina are a point further back in fifth after Adrian Mutu's brace set up a 2-1 win at Parma, although his second from the penalty spot late on followed a dive from Christian Vieri.