Earl managed to floor Katsidis momentarily with a heavy right hand at close quarters, rendering the latter visibly stunned and unsteady on his feet. Earl's comeback would ultimately be short-lived, as Katsidis recovered swiftly and finished the round strongly. At the very end of round three, a low blow cost Katsidis a point, but in the following rounds he went on to deliver a barrage of largely unanswered punches to Earl until the decision was made Earl's cornermen to stop the fight after the fifth round. United States debut Katsidis was scheduled to face then-WBO super featherweight champion Joan Guzmán on 26 May 2007, but the latter withdrew from the fight due to a hand injury. Katsidis later defended his WBO interim lightweight title against Czar Amonsot on 21 July 2007 at the Mandalay Bay Events Center in Las Vegas, Nevada. This was Katsidis' first fight in the United States, as well as the first of his many appearances on the HBO network, as part of the pay-per-view undercard to Bernard Hopkins vs. Ronald "Winky" Wright. Both boxers would go on to wage an intense, bloody war over the twelve-round distance. Amonsot proved to be a tough and durable opponent, despite suffering knockdowns in rounds two and ten. The judges' scorecards ultimately read 116–110, 115–111 and 114–112 for Katsidis, but both men were subsequently hospitalised at the local Valley Hospital Medical Center, with Amonsot requiring an overnight stay and Katsidis multiple stitches for serious cuts around his eyes, which had begun to bleed heavily from the third round onwards. Amonsot was later found to have a subdural hematoma, putting his career in serious doubt at the time. 2008 First defeat A bout with then-IBF/WBA/WBO lightweight champion Juan Díaz was to take place after the fight with Amonsot, but problems with HBO and Díaz's promoter Don King prevented this from occurring. On 22 March at the Morongo Casino, Resort & Spa in Cabazon, California, Katsidis fought reigning WBC and The Ring magazine lightweight champion Joel Casamayor. The Cuban southpaw got off to a good start by catching Katsidis early on with sharp left hands, dropping him twice in the first round. As the fight continued, Katsidis' harder punches and constant pressure began to find their way through Casamayor's defence, at one point sending him tumbling through the ring ropes towards the end of round six, after successive left and right hands