MARK Petchey of Britain upset an error-prone Michael Chang, the top seed, 7-5, 6-2 in the first round of the US$313,750 Los Angeles Open. Second-seeded Boris Becker of Germany showed no effects from a four-week lay-off and defeated Grant Stafford of Canada 7-5, 6-3 in the first round of the 32-player, hard court tournament. Third-seeded American Andre Agassi, fresh from winning the Canadian Open last week, eliminated Nicklas Kulti of Sweden 6-2, 6-1 in another first-round match. Jared Palmer upset fellow American Brad Gilbert, the seventh seed, 6-7 (5-7), 6-3, 6-4 in the only other surprise of the day. Chang, the runner-up here last year, struggled with his serve and his normally reliable passing shots, while Petchey added to his discomfort by covering the net like a blanket. Petchey, ranked 89th in the world, captured the only service break of the first set in the 11th game after holding three previous break points against Chang, whose serve wavered between inconsistent and weak the entire match. Chang, ranked sixth, won his serve only once in four second-set service games as Petchey repeatedly attacked the net and created problems for the American's passing shots. Chang gave credit to his opponent for taking advantage of his own poor play in the 72-minute match. ''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.'' Meanwhile, second-seeded Andrei Medvedev of Ukraine advanced to the second round of the US$365,000 Skoda Czech Open men's clay court tennis tournament in Prague. Medvedev, ranked seventh in the world, recovered decisively from a close first set to beat Peter Tramacchi of the US, 7-6 (8-6), 6-0. In other first-round matches, third-seeded Slava Dosedel of the Czech Republic defeated Alejo Mancisidor of Spain 6-3, 6-2, and seventh-seeded Karel Novacek, also of the Czech Republic, outplayed former champion Mats Wilander of Sweden, 7-5, 6-1. Novacek complained after the match that his performance was hampered because of the theft of all of his rackets, along with his car, earlier in the day. Other top seeds also advanced. Number four seed Andrei Chesnokov of Russia defeated Oleg Ogorodov of Uzbekistan, 6-4, 6-1; number six seed Alberto Costa of Spain got by Brazil's Jaime Oncins 6-3, 5-7, 6-3; and fifth-seeded Alex Corretja of Spain beat fellow countryman Felix Mantilla, 3-6, 6-4, 6-3.