"Grenadier"s fifth war patrol, under the command of LCDR John Allison Fitzgerald, between 1 January and 20 February 1943, brought her considerably better fortune than earlier patrols. A 75-ton schooner fell victim to her deck guns 10 January, and two days later "Grenadier" sighted a small tanker with a barge in tow. Judging the target not worth a torpedo, she slipped silently into the column behind the two Japanese ships. At dusk she battle surfaced. With binoculars lashed to the deck guns as sights, she raked the tanker and barge, sinking them immediately. The remainder of her patrol, along the Borneo coast through shallow and treacherous waters, was hampered by fathometer failures. She conducted an aggressive attack on two cargo ships 22 January but did not sink them.