When the WWF purchased WCW, the contract of Gruner (who was still wrestling as Billy Kidman) was one of twenty-five involved in the purchase. When The Invasion commenced, Gruner appeared on WWF television under his Billy Kidman ring name as a tweener for The Alliance. In July 2001, he won the Cruiserweight Championship from Gregory Helms in the first televised WCW match on "SmackDown!". As the WCW Cruiserweight Champion, Kidman proceeded to defeat the WWF Light Heavyweight Champion X-Pac at Invasion. After this, Kidman was later sidelined with an injury during the Invasion angle and thus did not have as big of an impact as other WCW wrestlers. On the October 11 episode of "SmackDown!", Kidman won his record-tying fifth Cruiserweight Championship after defeating X-Pac.