Alvarado made it 1-1 between the two warriors in front of 5,418 rabid fans at Mandalay Bay, with the judges scoring it 115-113, 115-113 and 114-113.
Rios’ stoppage of Alvarado on October 13 in Carson, California earned 2012 fight of the year honours from a host of media sources, including here at Eurosport-Yahoo!.
And following a rematch that was every bit as thrilling and physical as their first collision, both fighters immediately agreed a third bout was necessary to decide a winner in their trilogy.
"I won the first one, now they want to make the third one. Let's make the third one," Rios barked at Alvarado.
"Brandon gave me a shot to redeem myself," Alvarado said. "I'll give him a shot for the trilogy."
The two are already early front-runners to claim a second annual award for their war in the ring, along with Ruslan Provodnikov’s exciting defeat to Timothy Bradley.
The 32-year-old Alvarado and 26-year-old Rios have now become each other’s only professional career defeat, with Alvarado moving to 34-1 and Rios going down to 31-1-1. Each has earned 23 career knockouts.
On the undercard, Terence Crawford went 20-0 with a surprising decision win over Breidis Prescott – the man who handed Amir Khan his first pro defeat inside a minute – having taken on the fight at a fortnight’s notice.
Meanwhile a world title changed hands in Mexico, as Japan’s Katsunari Takayama (25-6, 10 KOs) claimed the IBF strawweight belt via unanimous decision over reigning champ Mario “Dragoncito” Rodriguez (15-7-4, 11 KOs at the Estadio Francisco Carranza Limon in Guasave, Sinaloa. Scores were 117-111, 119-109 and 115-113.
In the co-feature, Pedro Guevara (19-1-1, 13 KOs) won a split decision against former champion Raul “Rayito” Garcia (33-2-1, 21 KOs) to claim the vacant WBC 'silver' light flyweight belt.