Josh Sanders went from fighting to win "The Voice" to fighting for custody of his kids ... in what's shaping up to be an incredibly messy legal battle with his estranged wife.

Josh and his estranged wife, Kendra, have been going at it over their three children after separating back in January ... according to court documents obtained by TMZ.

In his court filings, Josh claims he should have custody because his ex is trying to turn the kids against him ... and says she's keeping him from co-parenting. He also claims one of the kids has said she wants to live with him.

But Kendra countered by alleging Josh is an "active alcoholic" who she says "drank away" his professional opportunities after coming in second on "The Voice" two years ago. 

She also claims he's driven drunk with the kids in the car and been violent in front of them.

Kendra argued that Josh is only fit for supervised visitation and insists she should have primary custody because she can "provide a stable, loving, alcohol free home" for the kids.

The whole thing is messy ... according to arrest records, Josh was arrested in April for violating a domestic violence temporary restraining order. And earlier this month, Josh filed a trespassing complaint against Kendra.

It's unclear what the domestic violence restraining order is about ... but the singer claims it was just about the custody dispute.