In May 2013, Tazawa was named interim closer after injuries to Joel Hanrahan and Andrew Bailey. By the end of July he had settled into the primary right-handed setup role ahead of new closer Koji Uehara. He continued in that role through Boston's run to the World Series championship, including pitching important innings in the ALCS and World Series. In 13 postseason appearances, Tazawa allowed only one run. He earned the win in the decisive Game 6 of the ALCS.