Rublev started his 2021 season with a title at the ATP Cup, a country-based team event. Playing for Russia, in a team with Daniil Medvedev, Aslan Karatsev, and Evgeny Donskoy, Rublev bested Argentina's Guido Pella and Japan's Yoshihito Nishioka in the group stage to help his team advance into the semifinals. There he defeated Germany's Jan-Lennard Struff, coming back from a set down to win 3–6, 6–1, 6–2. In the final of the ATP Cup, Rublev beat Italy's Fabio Fognini in straight sets.