But his life was unmasked in October 2009 when Czech Interpol called Auckland detectives with details of his true identity and an arrest warrant on charges in connection with the 2003 murder.
Now 30, Sroubek, who has New Zealand residency, faces deportation after being found guilty on false passport charges and making false statements to immigration officials.
He pleaded not guilty in the Auckland District Court but at trial admitted using a false identity.
His defence, led by David Jones, QC, was that Sroubek had a "reasonable excuse" to give a false name as he fled his homeland in fear of a criminal and corrupt police officers.
Sroubek told the jury about September 7, 2003 - the day Vladimir Domacka was shot in the head with a revolver in Prague. Sroubek was with a group of men when one, Stepan Cisar, smashed a bottle over another man. A scuffle broke out and Sroubek said he ran back to his car.
"I just didn't want to be a part of it, I had nothing to do with it."
Cisar then jumped into the car with him. "He said: 'Go, go, go, drive'. I said, 'What happened?' and he said: 'I think I just shot the guy'. Then I saw the gun in his hand," said Sroubek. "So of course I drove."
The next day Sroubek discovered Vladimir Domacka, a dodgy car dealer, was dead. He was called to a meeting with Cisar and two strangers, introduced as police officers. The pair said they could influence the investigation to make it look as if the gun fired accidentally during a struggle.
If he didn't co-operate, the police officers said Sroubek could be implicated in the murder and shot by police if he tried to run away.
His parents and close friends urged him to leave the country for his own safety, fellow kickboxer Jan Antolik giving Sroubek his own passport. He doctored the photo and fled to Germany, where he made a video statement about the murder and sent it back to Czech police.
A full transcript was given as evidence at Cisar's murder trial in December 2004, which convinced two other witnesses to come forward. The video was also played in the Auckland District Court to back up Sroubek's defence.
Cisar was convicted of murder, while the two other witnesses were given suspended sentences on charges of hooliganism and attempted bodily harm. An international arrest warrant for Sroubek still exists on those charges.
From his shaky start here eight years ago, Sroubek went on to represent New Zealand at international kickboxing under the ring name of Jan "Atomic". He now imports fruit beverages from Europe for supermarkets around the country.
In his closing address to the jury, Crown prosecutor David Johnstone said it was "indefensible" to suggest Sroubek's story was made up. "It clearly isn't."
But the two other witnesses at the murder trial still lived in the Czech Republic without any recrimination from the convicted mobster and received light sentences on their charges.
And despite fears of a corrupt police force, Mr Johnstone said the Czech investigation was conducted in a proper manner with a transcript of Sroubek's statement presented unchanged at Cisar's trial.
"[Sroubek] preferred to stay in New Zealand rather than face a fair trial."
Closing the defence case, Mr Jones asked the jury: "What level of fear would drive a mother and father to tell their son to leave home, the country, Europe, and go to the other side of the world?"
Sroubek had given "damning evidence" against an underworld figure but with no witness protection at home, he had to create a new identity.
"This case calls out for a reasonable excuse. He has told you everything."
But the jury found Sroubek guilty on all charges, stunning his parents who flew here to give evidence. He's been released on bail until sentencing next month.
Steve Stuart, Immigration NZ general manager intelligence, said getting immigration status through identity fraud was not tolerated.
"We are now reviewing this person's immigration file with a view to deporting him from New Zealand," he said.
The case
Real name: Karel Sroubek, born February 28, 1981.
False passport name: Jan Antolik, born October 20, 1981.
Guilty
April 2005: Without reasonable excuse, provided false information in applying for visitor visa and work permit.
November 2007: Without reasonable excuse, provided false information in applying for work permit and residency application.
August 2006-November 2009: Without reasonable excuse, was in possession of a false passport.
