Although Stevenson, a hard-punching southpaw, dominated the first seven rounds, during which he knocked Fonfara down with a left hand in the third round and again in the fifth round with a body shot, Fonfara stormed back in the later rounds of the fight.
He turned the tide in the eighth round and then knocked Stevenson down in the ninth round and had him in major trouble. Stevenson hung on to win a unanimous decision by scores of 116-109, 115-110 and 115-110.
Now they will meet in a rematch on Saturday (Showtime, 9 p.m. ET) at the Bell Centre in Montreal, the same arena as their first encounter.
In the co-feature, Montreal rivals Eleider "Storm" Alvarez (22-0, 11 KOs), 33, will risk his status as the mandatory challenger for the winner of the main event by taking on former light heavyweight world champion Jean Pascal (31-4-1, 18 KOs), 34, who is trying to keep his career as a meaningful fighter alive, in a 12-round bout.
After a life-and-death fight the first time around, Stevenson, 39, who will be making his eighth title defense, is well aware of what Fonfara brings to the table and is aiming for a knockout this time to leave no doubt.
"I'm ready because I know Fonfara is dangerous. We're not underestimating him," Stevenson said. "I've prepared for everything; everything he brings in the ring I'll be ready for it.
"I'm training for the knockout. (The late) Emanuel Steward (Stevenson's former trainer) always told me, 'Knockouts sell.' When I get in the ring I'm going for a knockout. It's not an option for me to go 12 rounds.
"I train for 12 rounds, and if it goes 12 rounds, he's going to get punishment the whole time, but I definitely am going for the knockout."
Stevenson (28-1, 23 KOs) is a gifted puncher. He won the title by a first-round knockout of Chad Dawson in June 2013 and has won five of his seven defenses by KO.
The 29-year-old Fonfara (29-4, 17 KOs), who is from Poland and fights out of Chicago, believes getting off to a better start than he did in the 2014 fight is pivotal.
"I can't make the same mistakes I did in our first fight," Fonfara said. "I must fight much smarter. Stevenson only has a good left hand. He's the champ, and he's a good fighter, but his boxing isn't amazing. He's not easy, but he only has basic boxing skills. We must cut (off) his left hand and be ready to throw my right. We need to control him.
"I must start good in the first round and let him know that I'm here to win the fight. And I'll be the new champion. I'm going to win the fight. A world title is the only thing I've never had. I've imagined raising my hand after the fight and becoming the new WBC champion. I'm ready to take the title. My time has come. I'm always a fighter that goes forward and pushes guys back, but I am now ready to fight backwards if I need to. I'll show the best Andrzej Fonfara this time.
"I respect him because he's the champ. I came here to show that this time, I'm the better fighter."
Since the loss to Stevenson, Fonfara has had his ups and downs. He won his next three fights, including making Julio Cesar Chavez Jr. quit in the ninth round and winning a brutal slugfest with Nathan Cleverly, who went on to win a light heavyweight belt for the second time.
But then he suffered a shocking first-round knockout against Joe Smith Jr. last June before rebounding to knock out Dawson in the 10th round on March 4, essentially ending Dawson's career.
Fonfara also began training with Virgil Hunter, who he believes brings a lot to his preparation.
"I'm a much smarter fighter now. Mentally, I'm much stronger and smarter," Fonfara said. "I bettered myself. Virgil has taught me a lot of new stuff. Some things worked in my last fight with Chad Dawson, some things didn't. But that was our first fight together. Now I'm ready to show everything in this fight.
"I've had a lot of success against southpaws. I like fighting them. Chad Dawson was a southpaw, so the last two fights, the last four months, we prepared only for southpaws. I'm ready for the best Stevenson. It's my second full camp with Virgil Hunter, and we've improved a lot since the first fight."
Stevenson's view is that it won't matter who Fonfara had training him or who he will have in his corner come fight night. He said he learned from the first fight and is not underestimating him like he may have done last time.
"I see some things I can exploit in the ring," Stevenson said. "I won the first fight, and I know him very well. I know what he can do. I know he changed trainers. Virgil Hunter brings a lot of experience. I know he trains (unified light heavyweight titleholder) Andre Ward, and I know he's very intelligent.
"I know Andrzej Fonfara from 12 rounds in the ring. He can't change his style right now. He may show some improved defense, but after a couple of rounds it will be the old Fonfara. This fight will be different. I expect a knockout. I hurt my hand in our first fight, but now I'm fully healthy, and I expect to knock him out. I know Fonfara is training hard to take my belt. I know he's a tough fighter, and I know it's not going to be easy for me."
Fonfara is hungry to take the title and Stevenson said he wants to win and move on to unify the division.
"I'm ready to be the new world champion, and I'll do that Saturday night. He wants the KO, and I want the KO, too," Fonfara said.
Stevenson said he will be watching the June 17 rematch between Ward and former unified titlist Sergey Kovalev closely. He turned down previous opportunities to unify titles with Kovalev before he lost the belts to Ward in November. Stevenson, however, still claims he wants to fight him or Ward -- whoever wins.
"My goal is to unify the titles with the winner of Kovalev and Ward. I want the unification, but if not, I'll make my mandatory," Stevenson said. "If Ward wins I want to fight him. If Kovalev wins I want to fight him. I'm ready."