In the Four Days Battle of 1666, Tjerk, now calling himself De Vries ("The Frisian"), was second in command in the squadron of the Zealandic Lieutenant-Admiral Cornelis Evertsen the Elder. When the latter was killed on the first day, Tjerk became the squadron commander, still using as flagship his Groot Frisia.
During the Four Days Battle of 1666, a man named De Vries ("The Frisian") held the second-highest position in Cornelis Evertsen the Elder's naval unit. When Evertsen was killed on day one, De Vries assumed command and retained his flagship, Groot Frisia.