He gained revenge for his G-Mex Centre misfortune with a near-perfect display against the durable German in Hanover, and claimed the World Boxing Organisation cruiserweight title.
"I knew from the first fight I was a world champion in all but name," Thompson said. "But it's nice to be finally recognised as a winner rather than a loser."
He dominated all 12 rounds but had to make do with a split decision after the Spanish judge, Jose Luis Sousa, somehow scored the bout 115-113 to Rocchigiani. His two American colleagues scored it 119-109 and 117-113 the other way.
The plan is for Thompson to make at least one voluntary defence and then take on Sheffield's European champion, Johnny Nelson.
By contrast, Cardiff's Nicky Piper will now be concentrating on his commentating career after succumbing to the WBO light-heavyweight champion, Dariusz Michalczewski. Piper was down twice in the opening two minutes and, apart from a brief flurry in the fifth round, he took a severe seven-round beating before quitting on his stool.
The European super-bantamweight champion, Spencer Oliver, will take a break following his victory over Italy's Vincenzo Belcastro at Alexandra Palace last night. The unbeaten Londoner's fourth fight in less than eight months, and his second title defence in 12 weeks, was gained at the expense of a cut eye and an injured hand.
Oliver, who has called off his proposed voluntary defence against Fabrice Benichou in November, said: "I'm going to have a rest until January. I damaged my left hand against Belcastro, and, having injured my right in beating Serge Poilblan in July, it's best to let the injuries heal."
Oliver's victory, his 13th in a row, came on a split decision, with the French judge, Jean-Francois Toupin, giving the 12-rounder to Belcastro by a point.
Belcastro's head opened a cut below Oliver's right eyelid in round seven, and it reopened in round nine and 11, but Oliver was always in control. He expects to return against the mandatory challenger Sergei Devakov, from Ukraine, who outpointed Britain's Carl Allen on the Alexandra Palace undercard.
Crawford Ashley, in contrast to Oliver, completely lost the plot - and his European light-heavyweight title - when he was knocked out by the Norwegian Ole Klemetsen inside two rounds. The Leeds fighter never recovered after being floored by a left hook in the first round. Another left-hander put him down at the end of the second, the referee Mickey Vann declaring Klemetsen the winner by a technical knockout.Reuse content