And boy, did they deliver. In a slugfest which will rank high on the Fight of the Year list and be remembered for a long time to come, Melendez and Sanchez went toe-to-toe for 15 minutes of insane action.
In the end, Melendez won via unanimous decision at Houston's Toyota Center, taking scores of 29-28, 30-27, and 29-28.
"That's what Mexicans do, we hold our ground and we fight," said Melendez (22-3). "I'd rather go down on my shield then run in circles."
While the first two rounds were action-packed, Melendez, the former Strikeforce and WEC champion, got the best of both.
Sanchez actually got Melendez in a choke while mounted on a standing Melendez's back early in the first round. But Melendez took over from there, and opened a nasty gash on Sanchez's left eyebrow with a standing elbow, from which he bled profusely for much of the fight.
Melendez got the best of round two. Sanchez was willing to eat several punches to land one good one. The strategy seemed to be backfiring, as Melendez turned up the heat and Sanchez appeared to be fading.
Then came five minutes of pure magic in round three.
The classic Diego Sanchez, the one who might not be the world's most purely talented fighter, but has as much heart as anyone who has ever competed in the Octagon, emerged. With the Toyota Center crowd firmly behind Sanchez, the duo wailed away on one another with wild punches, as if they were a pair of smaller Wanderlei Silvas.
We almost came to an early conclusion as the doctor checked Sanchez's cut. But he was cleared. And then Sanchez dropped Melendez with a huge uppercut and nearly finished the fight. He also got Melendez in a guillotine, but Melendez escaped.
"I don't know, I really didn't have much going through my mind," Sanchez (24-6) said. "We were so slippery it was hard to do any ground fighting or anything. We were two slimy lizards, I swear.
The two winged wild punches at one another right up until the final horn.
Melendez, who lost a controversial split decision to Benson Henderson in April in a title fight, won for the eighth time in his past nine fights.
"Diego's a warrior, I respect him so much," he said. "If I can get through him, I can get through anyone in the division. I think I'm the uncrowned champ, and I'm coming for that belt."
Sanchez, meanwhile, left no doubt about his desires.
"I want five rounds," he said. "I want a rematch."