Chael Sonnen defeated Quinton "Rampage" Jackson by unanimous decision in the Bellator 192 main event at The Forum in Inglewood, California, on Saturday night.
Sonnen scored a takedown in each round despite giving up 30 pounds to Jackson, and all three judges scored the fight 29-28 in Sonnen's favor.
The bout was a first-round match in the eight-man Heavyweight World Grand Prix to determine a new heavyweight champion. Sonnen moved on to the semifinals, where he will face the winner of the clash between Fedor Emelianenko and Frank Mir.
The 40-year-old Sonnen improved to 30-15-1. He is on a two-fight winning streak after beating Wanderlei Silva at Bellator 180 in June 2017.
Jackson, 39, fell to 37-13 and has lost consecutive fights following a Bellator 175 defeat at the hands of Muhammed Lawal in March 2017.
After losing to Rashad Evans at UFC 167 in 2013, Sonnen went more than three years before returning to action. He lost to Tito Ortiz in his promotional debut at Bellator 170 last year, but he now has plenty of momentum heading toward the semifinals with consecutive wins over name-brand fighters in Silva and Jackson.
After losing three consecutive UFC fights from 2011 to 2013, Jackson revitalized his career.
He was 5-1 in his previous six outings prior to Saturday's loss, but it's fair to question if he is slipping as he approaches the age of 40.
The former UFC light heavyweight champion has been a consistently active fighter for nearly two decades, and that could be taking a toll on him.
Although Jackson's Bellator future is somewhat unclear, Sonnen can begin preparing for the next step in becoming heavyweight champion.
Emelianenko and Mir are set to fight in April, with the winner of that bout taking on Sonnen for the right to compete for the Bellator heavyweight title, which has been vacant since May 2016.