Blake Shelton and his girlfriend, Gwen Stefani, are celebrating a joint milestone in their music careers. The pair's romantic duet from Shelton's recent Fully Loaded compilation album, "Nobody But You," has soared to the No. 1 position on Billboard's Country Airplay chart for the week ending May 2.

"Congratulations @gwenstefani on your first country song going #1 at country radio!!! Not bad for your first try," Shelton jokes on Twitter.

It's been a steady climb for the song, which the duo performed from home on The Tonight Show earlier this month. Over the last three weeks, "Nobody But You" has ascended the Country Airplay chart by one position per week. For the week ending April 25, it was No. 2, behind Gabby Barrett's "I Hope."

"I knew the moment I heard this song that I wanted to record it," Shelton says, before recognizing the team of songwriters behind the chart-topper: "Shane McAnally, Ross Copperman, Josh Osborne and Tommy Lee James, you wrote a perfect song in which every lyric has meaning in my life, and I thank you."

The singer continues, "Another thanks goes to the fans who continue to listen — and I know there are a lot of distractions these days. One day soon, we can all celebrate music together again."

Of course, Shelton has another person to whom he wants to express gratitude: his partner and musical collaborator, Stefani, who can now add the achievement of a country smash to her already well-decorated résumé. Shelton marked the accomplishment by adding, "Finally, thanks to Gwen for joining me and making this one of the most special songs I've recorded."

"Nobody But You" was first released as a single in January, a month after its parent album was issued. Since then, Shelton and Stefani have released an accompanying music video, dropped an acoustic version, and belted the number together at both award shows and during television specials.

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