Shamrock then made his first title defense against then undefeated Extreme Fighting champion Russian Igor Zinoviev at UFC 16. Zinoviev was a feared Russian kickboxer and sambo specialist who held wins over Mario Sperry and Enson Inoue. Shamrock shot a double leg takedown and slammed Zinoviev down so hard that it knocked him unconscious. Zinoviev suffered a broken collarbone and a fractured C-5 vertebra from the slam and had to be carried out on a stretcher. Zinoviev's fight with Shamrock forced him to retire permanently from mixed martial arts ending the Russian's career. Shamrock then defended his belt against Jeremy Horn at UFC 17, submitting him with a kneebar. In October 1998, Shamrock avenged his earlier loss to John Lober by beating him decisively in 7 minutes at "UFC Brazil".