The NFL player union is suing the league on behalf of Ezekiel Elliott, seeking to vacate the upcoming ruling of an arbitrator over the Dallas running back's six-game suspension in a domestic violence case.

The lawsuit was filed late Thursday night in federal court in Texas. It accuses the NFL's appeal process of being "fundamentally unfair" because arbitrator Harold Henderson denied attempts by Elliott's attorneys to have his ex-girlfriend testify at a hearing that wrapped up earlier Thursday.

Henderson is supposed to rule on the NFL's decision to suspend Elliott "as soon as practicable," according to the labor agreement. But the lawsuit seeks to pre-empt Henderson's ruling.

Elliott, the NFL's 2016 rushing leader as a rookie, was suspended after the league concluded he used physical force last summer in Ohio against Tiffany Thompson, his girlfriend at the time. Prosecutors didn't pursue the case, citing conflicting evidence. Elliott has denied the allegations.

The personal conduct policy was amended three years ago to stiffen penalties in domestic cases. The change came after NFL was sharply criticized for its handling of a case involving former Baltimore running back Ray Rice.