The 31-year-old star treated fans to an impromptu performance with friend Ashley Monroe at the 28-year-old singer's concert at The Basement East in Nashville.
The appearance came just two days after Lambert and Shelton announced they will divorce.
ABC News Radio producer Hunter Kelly caught the moment on video, which sees Monroe embrace her friend to audience applause.
The women performed Lambert's song "Heart Like Mine," which the pair co-wrote with Travis Howard.
Monroe was in concert to promote her third studio album, The Blade, which will debut in stores and online Friday. Lambert last released Platinum in June 2014, and is slated to perform a sold-out show at 3rd & Linsley in Nashville on Tuesday, July 28.
Lambert and Shelton met at the CMT 100 Greatest Duets concert in 2005, and married in Boerne, Texas, in May 2011.
Shelton broke his social media silence Tuesday, while singer Chris Young took to Twitter to deny rumors of an affair with Lambert.
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