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.
In the Four Days Battle of 1666, Tjerk, now known as De Vries ("The Frisian"), served as second-in-command in Cornelis Evertsen the Elder's squadron, Zealandic Lieutenant-Admiral. Following Evertsen's death on the first day, Tjerk took command of the squadron and continued to use his flagship, Groot Frisia.