Burly Brazilian forward Hulk scored Porto's match winner from the spot just before the half-time whistle after Sapunaru was brought down in the area by Guimaraes' Leonel Olimpio.
Porto played Guimaraes in the last three official matches, thrashed them 6-2 in May to claim the Portuguese Cup and won 2-1 a week ago to lift the Portuguese Super Cup.
Last season runners-up Benfica drew 2-2 at newly-promoted Gil Vicente after failing to defend an early two-goal lead.
Spanish forward Nolito and Argentine striker Javier Saviola scored one each for Benfica in the first 20 minutes before Gil Vicente striker Hugo Vieira pulled one back on 37 minutes.
Brazilian forward Laionel then completed Gil Vicente's comeback midway through the second half, levelling with a delightful long-distance effort.
Sporting drew 1-1 at home with Olhanense, leaving coach Domingos Paciencia disappointed with his first official match in charge of the Lisbon side.
In the first half, Olhanense striker Wilson Eduardo sent in a thunderous shot from outside the box that gave Portugal keeper Rui Patricio no chance. Russian substitute, Marat Izmailov, still toe-poked the 1-1 for Sporting on 77 minutes but they failed to score a winner despite several chances.
