Petchey, ranked No 89 in the world, took advantage of an error-prone display by the 1989 French Open champion to win 7-5, 6-2 in 72 minutes.
This latest victory is another indication that Petchey, who celebrated his 24th birthday on Monday, is capable of making real progress. Earlier in the year in South Africa he beat the former Wimbledon champion and world No 2, Michael Stich, and also pushed Andre Agassi to three sets in Key Biscayne.
'Once I got up a break in the first set, my confidence went up,' Petchey said. 'I was fresh today. I played pretty much as well as I could play. When I was up 3-0, I knew I had it. He obviously didn't play his best match, but I had to stay with him. He's capable of coming back.'
Chang, the runner-up here last year and No 6 in the world, struggled with his serve and his normally reliable passing shots, while Petchey added to his discomfort by dominating at the net.
'It's tough when you're not able to do what you want to do,' Chang said. 'Some of the shots I was hitting were missing by five feet. That's frustrating when you're playing a guy who's coming into the net and you only have the smallest margin for passing shots.' Petchey now faces the Australian, Mark Woodforde, in the second round.
In other first-round matches, Germany's Boris Becker, seeded No 2, beat Grant Stafford, of South Africa, 7-5, 6-3, while another American, Andre Agassi, fresh from his victory at the Canadian Open, overpowered Sweden's Nicklas Kulti 6-2, 6-1.
'Brad (Gilbert) has been helping more with strategy, the mental part,' Agassi said. 'It's giving me more consistency. But I still play my game, and it's nice to be on your game.'