The bout was supposed to be for the WBA lightweight title but Rios had tipped the scales two pounds over the 135lb limit.
And, despite being sluggish and scrappy throughout, Rios was judged to have won with scores of 116-112 and 115-113 in his favour; a decision that angered many fans at the Mandalay Bay Resort and Casino.
The third judge called the fight 117-111 in favour of Abril, who dominated several rounds with superior aggression and cleaner shots.
"Abril was an awkward fighter," said Rios. "He held a lot. I thought referee (Vic Drakulich) should have penalized him for holding. I felt good physically."
"Trying to make weight did not drain me. It was not my best fight but I did fight well. 140 lbs is where I am going - 140 here I come."
Rios’s next fight is likely to be against
Marquez, who also holds titles in featherweight, junior lightweight and lightweight divisions, won the bout with scores of 119-109, 118-110 and 118-110 to make amends for his narrow loss to Manny Pacquiao last November.