Andrzej Fonfara knocked down Julio Cesar Chavez Jr. in the ninth round and won after the bell Saturday night when the referee stopped it.
Fonfara (26-4, 15 KOs) landed big shots all night at the outdoor StubHub Center ring, and his left hand in the ninth knocked down Chavez (48-2) for the first time in the boxing scion's career.
The referee ended the bout on advice from Chavez's trainer, Joe Goossen. The crowd threw beer while roaring its displeasure, and Chavez claimed in his post-fight interview that he thought he was winning.
Fonfara, a native of Poland living in Chicago, was penalized a point for using his shoulder in the seventh, but appeared to be in control before the stoppage. The win was his second straight since a competitive loss to light-heavyweight champion Adonis Stevenson.
The enigmatic Chavez hadn't fought since March 2014, when he convincingly won a rematch with Bryan Vera. Chavez took his only previous loss in September 2012 during his memorable brawl with middleweight champion Sergio Martinez.