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". He specially fought well on the last, fourth, day, strongly contributing to the Dutch victory. Six weeks later during the St James's Day Battle he was second in command of the van under Lieutenant-Admiral Johan Evertsen. When this squadron failed to reform a proper keel line after a calm, and was mauled by the line of Admiral Rupert of the Rhine. Tjerk had an arm and a leg shot off; yet still in vain tried to rally his force. His crippled ship drifted away, only discovered by the Dutch rear under Cornelis Tromp the next day. The wounded Frisian admiral was speedily brought ashore in Flushing by a yacht - but died from his wounds on his birthday, 6 August 1666.