Blake Shelton is already building a playful rivalry with new "The Voice" coach Nick Jonas. The 43-year-old country music star and "The Voice" coach razzed Jonas, 27, while discussing Season 18 of "The Voice" during Friday's episode of "The Ellen DeGeneres Show." Jonas will replace Shelton's girlfriend, singer Gwen Stefani, as a coach in the new season. "The Voice" is in the midst of its 17th season on NBC and will return for Season 18 in February. "We've actually already filmed the blind auditions for Season 18. So I have had a chance to work with him," Shelton said of Jonas. "I had met him a couple times before. Super nice guy -- he's a sucky coach. I'm going to tell you right now -- terrible," he jokingly added. "Horrible coach." Shelton previously had a rivalry with Maroon 5 singer Adam Levine, who left "The Voice" after Season 16. Shelton predicted that one of his team members will win Season 17, but suggested Stefani's contestant Rose Short as an alternate. Shelton has been dating Stefani since 2015, and called the singer the "sexiest" coach on "The Voice." Host Ellen DeGeneres encouraged Shelton to propose to Stefani after previously giving Shelton an engagement countdown clock. Shelton and Stefani released the new duet "Nobody But You" on Thursday. The song appears on Shelton's album Fully Loaded: God's Country, released Friday.