On 21 December 1941, "Ro-33" got underway from Cam Ranh Bay to begin her first war patrol, assigned an operating area in the South China Sea east of Singapore in support of the Japanese invasion of British Malaya. Her patrol was uneventful, and she departed her patrol area on 5 January 1942 to return to Cam Ranh Bay, where she arrived on 7 January 1942.