Chris Stapleton and Miranda Lambert

Watch Miranda Lambert Join Chris Stapleton for Live Performance of 'Fire Away'

Chris Stapleton and Miranda Lambert joined forces over the weekend to sing an incredible rendition of his hit song "Fire Away."

Lambert's boyfriend, Anderson East, has been opening for Stapleton's All-American Road Show Tour. On Sunday night (July 16), Lambert popped in to the Buffalo, N.Y. show to sing happy birthday to Anderson onstage. But that wasn't her only appearance during the evening's festivities.

Welcomed by screaming fans, Lambert joined Stapleton and his wife, Morgane, to perform "Fire Away." Although Miranda initially kept things restrained, she showed off her powerhouse voice during the final reprise of the chorus.

After postponing a few shows due to a hand injury, Stapleton is back on the road. His Traveller has sold more than 2 million units. The album won a slew of awards last year, including a Grammy for Best Country Album of the Year and the Billboard Music Award for Top Country Album. Likewise, Stapleton's most recent album, From a Room: Vol. 1, had the largest opening sales week for a country album in a year and a half, the first since Luke Bryan's Crash My Party.

Chris and Morgane Stapleton have upcoming tour dates scheduled in New Jersey, New York, and Virginia.

Now Watch: Inside Chris and Morgane Stapleton's Incredible Love Story