The eight-time Grand Slam winner shared a sponsor with Armstrong for much of his professional career and admitted that he did not believe he had cheated until hearing his confession during an interview with Oprah Winfrey last week.
He told reporters at the Australian Open earlier today: "My reaction to it is the same as everybody. It was shock, hard to stomach, sadness, disappointment. I think anger is a fair word.
"I was certainly one of those that flat out believed him that long period of time. The thought of it not being the case was unconscionable to me.
"My next reaction went to Livestrong and all the people it helps, people fighting for their lives, survivors of real battles, and I hope for its survival through this. I hope Livestrong can survive this."
Agassi is in Melbourne for the first time since his retirement in 2006 and will present the men's singles title following Sunday's final at Melbourne Park.