Cort Sivertsen was born in Brevik, Norway, the son of a shipper. At the age of fifteen he took service with the Dutch navy; in 1639 he fought under Lieutenant-Admiral Maarten Tromp at the Battle of the Downs. In 1642 he was first mate on the "Grote St. Joris", a Dutch ship hired by the fleet of Venice as the "San Giorgio Grande". Sivertsen called himself "Adelborst" in this period, a Dutch name meaning "cadet". In 1645 he became captain of the "San Giorgio" and entered full Venetian service. In 1650 the "San Giorgio" became a flagship and Sivertsen therefore flag captain. In an action against the Turkish fleet on 13 May 1654 near the Dardanelles he broke with his ship a line of Turkish galleys and sank fifteen of them; next day the Turkish garrison of Tenedos surrendered to him. For this he was knighted in the Order of Saint Mark; the Venetian senate rewarded him with an annuity of 1400 golden ducats. In 1660 he was made Vice-Admiral.