Jorge Jesus' men failed to win their first league game of the season for the ninth year in succession as they went down 2-1 to Maritimo at Estadio dos Barreiros.
Derley opened the scoring for the hosts in added time in the first half and, although Rodrigo replied early in the second for Benfica, Sami wrapped up the win in the 78th minute.
The visitors suffered just one defeat in the league last season, but have now equalled that tally with just one game of the 2013-14 campaign completed.
Reigning champions Porto earned their 27th Primeira Liga success last term courtesy of going the entire season without losing, and extended their overall unbeaten run to 44 top-flight games with a 3-1 win at Vitoria Setubal.
The hosts held a deserved half-time lead courtesy of Rafael Martins' 14th-minute goal, but the game would descend into controversy early in the second period.
Porto were correctly awarded a penalty that Josue converted in the 49th minute, but Vitoria goalkeeper Pawel Kieszek would take exception to the midfielder's attempts to retrieve the ball and subsequently saw red after attempting to land a headbutt.
Paulo Fonseca's team made the most of their one-man advantage from that point on, as goals from Juan Quintero and Jackson Martinez wrapped up a routine win.
Despite that convincing victory for Porto, Sporting claimed the early lead in the Portuguese top flight as they thrashed new-boys Arouca 5-1 at home on Sunday.
A debut hat-trick for Seattle Sounders loanee Fredy Montero allied to goals from Mauricio and Wilson Eduardo helped Sporting claim an encouraging early win in their opening fixture.
Elsewhere, newly promoted Belenenses suffered a 3-0 defeat at home to Rio Ave, who had three different scorers to thank for their comfortable victory.
And Gil Vicente beat Academica 2-0 at Estadio Cidade de Barcelos with Bruno Moraes converting a sixth-minute penalty and Luis Martins calming home supporters' nerves in the fourth minute of added time.
