Isaac Lucero’s aggression toward the end of the eighth round paid dividends Saturday night.

The entertaining Mexican junior middleweight went after a fading Roberto Valenzuela and landed several flush, punishing punches that made referee Mark Nelson step between them to stop their scheduled 10-round fight at Frost Bank Center in San Antonio. Valenzuela protested once Nelson ended their fan-friendly slugfest with one second to go in the eighth round on the Isaac Cruz-Lamont Roach undercard.

Mexico’s Valenzuela (31-6, 29 KOs) took Lucero’s power well before that pivotal point.

Lucero (18-0, 14 KOs), 27, has won seven consecutive bouts by knockout. Valenzuela, 26, lost inside the distance for the third time in his eight-year career.

In the round before he bombarded Valenzuela with shots to end it, Lucero blasted Valenzuela with several flush rights.

Lucero unloaded various power shots on Valenzuela in the first half of the sixth round. Valenzuela withstood Lucero’s onslaught and caught his opponent with uppercuts and hooks.

After another back-and-forth fourth round, Lucero landed numerous right hands during a fifth round which built some momentum. Switching to a southpaw stance helped Lucero succeed in the third round, particularly with straight lefts.

Valenzuela and Lucero caught each other with numerous power punches in the second. Valenzuela exposed Lucero’s defensive flaws in those three minutes, including three left uppercuts that got the judges’ attention.

A right hand by Lucero sent Valenzuela to one knee with 57 seconds to go in the first. Valenzuela quickly complained to Nelson that Lucero hit him on the back of his head, but a knockdown was counted.

In the bout before Lucero’s victory, Dominican featherweight contender Luis Nunez remained unbeaten by unanimously outpointing Hector Sosa in their 10-rounder.

Nunez’s stiff jab consistently set up his offense, mostly left hooks up top, and kept the game, aggressive Argentinean from mounting enough offense to make their fight more competitive on two of the scorecards. Judges Jesse Reyes (98-92), Ignacio Robles (96-94) and Jose Torres (97-93) scored their fight for Nunez, who improved to 22-0.

Sosa occasionally caught Nunez with right hands, but he could never hurt him. The durable underdog pressed the action right up until the final bell, yet Sosa (18-4, 9 KOs) still lost a unanimous decision for the fourth time in his past nine fights.