A capella group Pentatonix are set to become a foursome after musician Avi Kaplan announced his impending departure.

The singer broke the news to fans in an emotional video post on Facebook on Friday (12May17).

"I have an announcement to make," Avi began the clip, in which he sits in the middle of his bandmates. "This decision is something I have been struggling with a lot and it's been one of the toughest decisions to make in my entire life. I've decided to take a step back from Pentatonix."

Fighting back tears, he went on to insist his six-year career with the band has been "the most unbelievable years of my life", but admits he has always struggled to keep up with the group's fast pace on the road to success, after winning talent show The Sing-Off in 2011 and shooting to fame.

Avi also confessed he had not adjusted well to being away from family and friends for long periods of time, and decided to walk away from the line-up because he didn't want his personal issues to hold Pentatonix back from continued success.

However, he plans to fulfill his commitments to the band's upcoming world tour, which will serve as his farewell to fans, reports TMZ.com.

Avi and his Pentatonix bandmates Scott Hoying, Mitch Grassi, Kirstin Maldonado, and Kevin Olusola, will kick off the shows in Japan later this month (May17), before returning home for gigs in Los Angeles; Columbus, Ohio; Des Moines, Iowa, and St. Louis, Missouri, among others.

The departure news emerges months after the musicians won the Best Country Duo/Group Performance Grammy Award for their collaborative remix of Dolly Parton's 1973 classic Jolene, which Pentatonix sang at the prestigious ceremony in February (17).