Serrano (13-2) began throwing devastating blows in the early rounds. Ambriz withstood the flurry of shots to the head and body. Ambriz battled back and took over the rest of the way.
The biggest hit of the evening came in the final MMA fight. Irvine's Luis Salgado caught Ozzie Alvarez from Miami, Fla., with an uppercut, sending Alvarez down.
Officials stopped the 185-pound fight in the third and final round at the 2:35 mark, when Salgado unleashed his shot. Salgado improved to 2-0, while Alvarez fell to 1-2.
The other knockout happened in the cruiserweight boxing match. Brent Urban of Dallas, Texas, floored Joseph Jones of Los Angeles in the second round at the 2:02 mark.
Urban (7-3-1) picked up his fifth knockout of his career. Jones dropped to 4-4.
The boxing fight that featured left-handed Oscar Andrade in the lightweight division also saw him remain undefeated.
Andrade, who is from East Los Angeles, recorded a unanimous decision against Los Angeles' Manuel Delcid, as each judge scored the four-round fight, 40-36. Andrade improved to 5-0 and Delcid is now 4-6.
The heavyweight division showcased a former University of Miami football player in the ring.
Quadtrine Hill, the ex-running back, and Chad Davis of Phoenix, Ariz., went all four rounds, but Hill recorded a unanimous decision and improved to 4-1. Davis fell to 2-8.
In the bantamweight division, Arizona's Jensen Ramirez was unable to record his first victory. The fight between Ramirez (0-1-1) and Los Angeles' Danny Roman (1-0-1) was scored a draw.
In MMA action, Carlos Garces of Huntington Beach fell to Isaac Gutierrez of Downey in a three-round 150-pound match. Gutierrez won by unanimous division.