It cost his newspaper a thumping $1.6 million in legal costs.
He was the subject of vilification from both cycling fans and officials -- not to mention from a man who had become a global sporting icon.
But not once did David Walsh waver in his quest to unveil the truth about Lance Armstrong and his doping lies -- despite the almost insurmountable obstacles placed in his way.
"People find this strange, but for me it was the time of my life," Walsh told CNN's Changing Gear series.
"I loved it. I thought 'This was journalism, this was what our game was about' -- asking questions that people didn't want to answer was actually the life blood of journalism for me at that time."
The award-winning Sunday Times journalist spent 13 long and sometimes lonely years pursuing Armstrong.
Labeled a "Little Troll" by the American, Walsh was finally vindicated when the disgraced cyclist confessed earlier this year.
It was Walsh's finest hour and he even gained an apology from Armstrong during the course of the Texan's televised interview with Oprah Winfrey earlier this year.
But the 58-year-old Irishman did not feel any sense of real elation -- only the satisfaction of a job well done.
"For me, it wasn't a conclusion that was thrilling, or exciting, or interesting -- it was the chase," said Walsh.
"I've used the expression that the hunt was better than the kill, and it very much was. I loved getting new information about Armstrong."
Back in 1999, when the world was in thrall to the cancer survivor after Armstrong's remarkable comeback to win the Tour de France that year, Walsh was immediately skeptical.
"It was perfectly obvious to anybody with half a brain that Armstrong was cheating," said Walsh.
"We were told this is a clean Tour, but it was the fastest in history -- completely illogical!"
Armstrong's treatment of the French rider Christophe Bassons, who was renowned for his anti-doping stance, only served to further raise Walsh's suspicions.
"I mean, Armstrong bullied him, most of the peloton bullied him and I thought: 'If you were anti-doping, that's not how you would treat somebody who clearly was riding clean.'"
It was the start of a crusade to uncover the truth, though at first nobody wanted to listen.
In the aftermath of the 1998 Tour de France -- blighted by doping, with police raiding teams to find illegal products -- cycling had been desperate for a good news story and was prepared, in Walsh's words, to "suspend disbelief."
"An American guy comes from Texas, single-parent family, he's come through life-threatening cancer, he's in the lead," explained Walsh.
"It's a story that could take the Tour de France from its knees and put it standing up again and the race organizers embraced that."
As Armstrong's winning run continued -- eventually to total an unprecedented seven Tour wins in a row -- so in Walsh's view the American's web of deceit grew and he alleges others were complicit in the cover up.
Walsh was determined to publish his version of events and when LA (Lance Armstrong) Confidential hit the bookshelves in 2004 and a story based on it was published in the Sunday Times, the libel suits and the threats intensified.
Walsh was at an explosive Tour media conference later that year and Armstrong, when inevitably asked about the book, singled him out, as the journalist vividly recalls.
Amrstrong said: "Well as the esteemed author is here, I will answer this.
"And then he said 'Extraordinary allegations, no, extraordinary accusations must be followed by extraordinary proof.' Everybody thought that was a great one-liner."
Walsh had based his book on interviews with Betsy Andreu, the wife of cyclist Frankie Andreu, a former teammate of Armstrong, Emma O'Reilly, who had acted as personal masseuse for the now disgraced cyclist, and a former teammate from the 1990's, Steven Swart.
When details of the systematic doping carried out by Armstrong and his team finally emerged in a report by the United States Anti Doping Agency (USADA) in 2012, it vindicated the whistleblowers' stance -- as well as Walsh and his co-author Frenchman Pierre Ballester.
But back in 2004, with Armstrong at the peak of his fame, it was inevitable he would challenge such damaging revelations to protect his reputation.
The Sunday Times stood by their man and his story, but when libel action in the UK courts was commenced by the litigious Armstrong, they knew the outcome would probably be in the American's favor.
Walsh recognized the seriousness of the situation, but admitted his judgment became clouded.
"I'm there saying 'Well I don't care, I just want this stuff out there.'
"And I wasn't seeing reason to be honest. I would have been a bit of a nightmare from the legal department's point of view and they were right.
"It did cost the Sunday Times a million pounds, but the newspaper were tremendously supportive as was my sports editor," added Walsh, referring to the out-of-court settlement reached with Armstrong in 2006.
By then the Texan had retired from the sport for the first time, though the rumors would not go away.
Walsh believes that cycling's governing body -- the International Cycling Union -- bears a heavy responsibility for not cleaning up its own sport in the face of overwhelming evidence of doping, not just by Armstrong but other leading riders.