The end came at 1:32 of the third round as referee John McCarthy called off the heavyweight fight after an exhausted Harris (2-1) collapsed to the canvas following a punch from Slice (6-2) that didn't appear to connect.
Harris, 38, who hasn't fought professionally since 2011, landed a fair share of punches in the bout, but he was visibly winded early in the first round. The 42-year-old Slice didn't look much better. The two leaned on one another, exhausted, throughout. McCarthy was forced to separate them numerous times due to inactivity. At one point in the second round, McCarthy stood Slice up from full mount.
The finish came under a chorus of boos. Harris was so tired he couldn't raise his arms, which allowed a lumbering Slice to take free shots at his chin. Harris was never knocked down from any of the shots, but the culmination of them eventually took an effect and he collapsed awkwardly in a heap. The Miami native was taken out of the cage by stretcher, but Bellator officials said he was able to sit and receive oxygen shortly after going backstage.
Slice, who is also from Miami, could barely stand during his post-fight interview. He showed no cosmetic signs of damage, but did absorb several left hands from Harris.
"I wish it could have been a little different," Slice said. "But hard work and the victory was ours."
The fight actually went to the ground numerous times, either by Slice takedowns or essentially by Harris willingly falling over to get a rest. By the second round, the 265-pound Harris could barely throw a punch, and the ones he managed seemed to have very little on them.
Slice is now 2-0 in Bellator. He earned a first-round knockout over Ken Shamrock in his promotional debut in June. Harris, the focus of a 2015-released documentary "Dawg Fight," suffered his first professional loss.
Campos knocks out Guillard
Campos nearly knocked Guillard out in the opening seconds of the fight, as he dropped him during an exchange in the pocket. Guillard eventually survived after being taken down and got his jab going. The former UFC veteran never recovered after getting hit again early the next round, and appeared out on his feet for a while before the stoppage.
Campos snaps a two-fight skid. Guillard, 32, is now 2-5 with one no contest in his last seven appearances.
Vassell outlasts Newton in rematch
Light heavyweight Linton Vassell (16-5) defeated former champion Emanuel Newton (25-10-1) by unanimous decision (30-26, 29-27, 29-27) in a rematch of their 2014 title fight. Newton won their bout in the final round with a rear-naked choke, but was mostly shut down in the second meeting.
Vassell moved to full mount in the first and third rounds. He flattened out Newton and landed hard punches to the side of his head. In the middle round, Newton was docked a point for kneeing Vassell in the groin.
The win improves Vassell's career mark in Bellator to 5-2. Newton has now lost three in a row.
Sanchez takes decision from Pineda
Sanchez, 25, struggled with Pineda's offensive grappling at times, but unquestionably landed the more efficient offense in the 150-pound catchweight bout. He scored punches from top position in the second and third rounds and hurt Pineda late on the feet. Pineda's right eye was badly swollen by the end.
Sanchez moves to 5-1 in the Bellator cage. His only loss in the promotion came against former champion Pat Curran.