Agassi was defaulted after he swore three times in front of a linesman during his second-round match on Wednesday night against his American compatriot Cecil Mamiit. At the time Agassi was 5-0 down in a second set tie-break, having won the first set 6-0.
The linesman told the umpire, Steve Ullrich, that Agassi had uttered audible obscenities in his direction twice, but Agassi denied directing it at anyone but himself and said he deliberately kept his hand over his mouth to muffle his response to frustration with his poor play.
"I'm shocked." Agassi said. "I didn't swear at him and I can't believe it happened. I never made eye contact with him and I never said the word `you.'"
Earlier in the set, Agassi swore at Mamiit because his opponent caught one of his errant serves and did not give the ball to the ball-boy, but a linesman apparently failed to hear him.
Unable to close out the match against a former national collegiate champion, Agassi grew visibly upset in the tie-break and finally lost his cool, ending with a swearing fit.
"It was a bad decision on my part but it was done in the heat of battle," Agassi said. "By the same token it was a bad decision on the umpire and lineperson's part. There's a need to keep things tight out there and I understand that but when there's not an apparent defacing of the game there's no reason to end the match that way. I was shocked, I wouldn't think in a million years how that would transpire."
Agassi, world No 7 and defending champion, will forfeit his prize-money and points from the event and could be fined up to $20,000 (pounds 12,500).
This was not the first time the sometimes temperamental American has been thrown out of an event. He was defaulted out of a tournament in Indianapolis in 1996 for swearing.
"I felt I was cheated," Mamiit said. "I was playing well. I wanted to find out the end result. I wanted to continue."
Chang will face the Norwegian Christian Ruud in the quarter-finals, who overcame the young American Paul Goldstein 6-1, 7-6.
Ruud, who unexpectedly beat world No 2 Alex Corretja at the Australian Open, said that his game has improved markedly since he took on a new coach, Thomas Hogstedt, and began practicing with leading Swedish players Thomas Enqvist and Jonas Bjorkman in Stockholm.
"They give me a lot more pace and when I used to practice in Norway, there wasn't anyone who could play at a top level," said Ruud, who at 66th is the highest-ranked Norwegian in history.
After battering Goldstein with heavy, deep groundstrokes, Ruud - who upset the fifth seed, Mariano Puerta, in the first round - said he could feel himself moving into good form and is looking forward to the French Open. "My goal is to reach the top 30 this year." he said. "If I do well the rest of the hardcourt season and reach the second week of Roland Garros, I have a good chance."
The eighth seed, Ramon Delgado of Paraguay, won his first-round match 5-7, 6-4, 7-5 over New Zealand's Brett Steven. Delgado is best known for his upset of Pete Sampras at the 1998 French Open.