[10][11][12] On 17 February 2007, a bout for the newly created and vacant WBO interim lightweight title was made between Katsidis and Graham Earl at the Wembley Arena in London.

[13] 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.

[14] 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. Winky Wright.

[17] On 22 March at the Morongo Casino, Resort & Spa in Cabazon, California, Katsidis fought reigning WBC and The Ring magazine lightweight champion Joel Casamayor.

All of this enabled Katsidis to gradually build up a lead on two of the judges' scorecards, until a well-placed counter left hand from Casamayor caught him on the chin in the tenth round, flooring him for a fourth time.

Barely able to stand up, Katsidis was able to carry on but shortly afterwards a further flurry of unanswered shots from Casamayor forced referee Jon Schorle to stop the fight.

Following the defeat, a rescheduled fight with Juan Díaz was set for 6 September at the Toyota Center in Houston, Texas, for the vacant IBO lightweight championship.

Over the course of twelve rounds, Katsidis underperformed and enabled a determined Díaz (who had himself suffered a career first loss in his previous fight) to counterpunch and time his attacks with regularity.

[22] Katsidis' next opponent was Vicente Escobedo, whom he fought on 19 September at the MGM Grand Garden Arena in Las Vegas, as part of the undercard to Floyd Mayweather Jr. vs. Juan Manuel Márquez.

This was dependent on whether Márquez was willing return to the lightweight division to defend his WBO championship, or if he would voluntarily relinquish it to Katsidis in favour of moving up to light-welterweight for potentially bigger fights.

[27] One day prior to Guerrero's withdrawal, Katsidis had turned his attention towards old nemesis Juan Díaz, with both expressing an interest in a potential rematch.

In the first two evenly split rounds, the occasional flurry of hooks from a highly aggressive Katsidis was enough to make Mitchell fight consistently on the back foot in an attempt to keep out of range.

From thereon, Mitchell was unable to fully regain his composure and, less than two minutes later, he was buckled by a hard left hook and a further succession of unanswered punches, at which point referee Dave Parris stopped the fight.

[33] Immediately after the Mitchell bout, the WBO mandated that Juan Manuel Márquez must face Katsidis for the 'full' version of the lightweight world championship, or risk being stripped.

[36] On 25 August the CEO of Golden Boy Promotions, Richard Schaefer, announced that Márquez had decided to stay at lightweight and defend his unified WBA, WBO, Ring, and lineal titles against Katsidis on 27 November at the MGM Grand.

He managed to recover swiftly, building up a lead on all the judges' scorecards and gradually neutralising Katsidis' aggression with accurate counterpunching at close range.

Guerrero was able to maintain a safe distance by consistently landing accurate punches with his jab and straight left hand, to which Katsidis had virtually no answer at any stage in the fight.

In the second round, Katsidis seemingly scored a knockdown due to a cuffing left hook, but this was not counted by referee Russell Mora.

[51] Having hired a new trainer in fellow Australian and former heavyweight contender Justin Fortune, Katsidis moved up light-welterweight again for a fight against Albert Mensah, which took place on 13 April 2012 at the Hard Rock Hotel and Casino in Las Vegas.

[54][55] Later in the year, Katsidis was scheduled to fight then-undefeated Darleys Pérez on 10 August at the Morongo Casino,[56] but later withdrew on 25 July due to a knee injury.

[58] In another in-depth December interview, Katsidis revealed that he had moved back to Sydney and hired veteran trainer Johnny Lewis for future bouts.

[59] However, on 12 February the news broke that the fight had been cancelled, with Katsidis having received medical advice based on MRI and CAT scans that he should retire.

[72] Katsidis is of Greek descent,[1] and often paid homage to his heritage by wearing a Corinthian helmet when entering the ring, as well as his trunks often resembling a warrior's skirt.