The 170-pound pair will meet Nov. 22 in Bethlehem, Pa., for $100,000 and the right to fight for the promotion's welterweight title.
Hawn bested Brent Weedman (30-27 on all three scorecards) on the strength of his sharp boxing and defensive grappling. ESPN.com scored the fight for Hawn, 29-28, giving Weedman a close third round.
"I landed more shots than I thought," said Hawn, 37, who pushed his record to 17-2. "The kid is tough, man. I was hitting him with everything I got; he wouldn't go down. I'm happy to get the win."
In a rematch of a tournament final at 155 pounds from May 2012, Hawn out-landed Weedman when it counted. The previous win allowed Hawn an unsuccessful challenge against Bellator champion Michael Chandler in January, which precipitated a move back up to the welterweight division.
The Olympic judoka strung together crisp combinations and counters, popping Weedman's head on multiple occasions. In the first frame, a right-left counter caught Weedman, 29, on the chin. That was the most meaningful exchange in the first five minutes. Action was sporadic, even if Weedman kept up his pursuit throughout the three-round contest.
When Weedman (22-9-1) mixed in kicks to the body, he scored. But Hawn kept control with his hands, and by the end, the younger fighter's face was noticeably marked up.
"It was an awesome performance," said Keslar, who stands almost a head taller than the 5-foot-9 Hawn. "I'm excited to take him on. He's an Olympian. He has competed at the highest level. I'm looking forward to facing him."
The November contest will be the third time Hawn has made it to a Bellator tournament final. He fell short in his attempt at welterweight in 2011, losing a split decision to Jay Hieron.
Keslar submits War Machine
Keslar entered the tournament as a late replacement and fought his way into the semis on the strength of a split decision over Sergio Junior. Unlike in that quarterfinal contest, Keslar had no need to test his cardio against War Machine, the controversial and colorful fighter who claimed after advancing in September that "this tournament is mine."
Training out of American Kickboxing Academy in San Jose, Calif., Keslar (11-3) credited his teammates and coaches for his success Friday.
"It feels great," he said. "This whole experience is the greatest opportunity of my life. I just don't ever want to let it go."
Keslar, 33, dragged War Machine, 31, to the mat early, setting the tone with an unrelenting body-triangle that kept him on his opponent's back all the way to the end.
Bleeding from the bridge of his nose, War Machine made an effort to stand, which he eventually did with Keslar suspended from his back.
Keslar waited for an opening to sneak in an arm under his foe's chin. The moment that happened, Keslar clasped his hands together and squeezed, and War Machine (14-5) went quiet.
"As soon as he stood up, I knew I'd be able to get his chin. As soon as I was able to lock it in, that [body] triangle was too tight for him to get out. But take nothing away from War Machine. The sucker is tough," Keslar said.
Grove wins Bellator debut
The scores read 30-27 twice and 29-28. ESPN.com saw the bout 29-28 for the 30-year-old Hawaiian.
"Give credit to Joe. Much more than I expected," said Grove, best known for his days fighting in the UFC. "I got winded trying to defend takedowns. He has good pressure. Hats off to him. But I felt I won that fight. Yeah, he took me down, but look at our faces. I did effective strikes. I was looking for submissions on the ground. Hats off to him. I couldn't catch him. I felt I got close. He's a true veteran."
Vedepo, 30, pressed for takedowns throughout the three-round bout, although Grove did well to defend, counter and neutralize the Cedar Rapids-based fighter's wrestling advances. With Grove (19-13) owning a 5-inch height advantage, it's no surprise he found pay dirt when he was able to lock up with Vedepo (14-7) and work an offensive clinch game. Elbows on the inside and, to a much larger degree, knees to the head helped Grove pull away as the fight progressed.
"Kendall Grove is a tough-ass dude," said Vedepo, who split his previous six contests heading into this match. "I'm glad I got to spend this time with him. I hope I fight more guys like him. I feel like I'm just getting started. I'm learning a lot. I feel I've conquered some of the things that have been plaguing me with my mind. I'm only going to get better from here on out."
The victory was Grove's first for Bellator and evened his 2013 campaign at 2-2.
Graham outlasts Prindle by decision
Judges at cageside called it a shutout for Graham (30-27, 30-27, 30-26), who repeatedly landed on his bigger, slower, defense-deficient opponent.
Graham, 38, made the fight his early, forcing Prindle, 37, to the floor with a series of right hands in the opening round. Graham followed Prindle to the ground and uncorked an elbow that opened a significant gash, prompting a visit from the cageside physician. The fight was allowed to continue.
Despite scoring several times inside with elbows, Graham began to slow midway through the second round. The contest was poor the rest of the way, with Prindle (8-4) doing little outside of taking whatever offense Graham was able to unleash.
The Australian K-1 veteran moved to 10-5 in MMA competition, ripping off nine straight wins after a 1-5 start. Prindle dropped his third straight contest under the Bellator banner.