ST. PETERSBURG, FLA., NOV. 30 -- Richard Fromberg had never played a Davis Cup match. He had never gotten beyond the second round of Wimbledon. Or the French Open. Or the Australian Open.

In fact, the 20-year-old, curly-haired Australian had never won a professional tennis tournament outside of Bologna or Bastad.

Tonight, American Andre Agassi frustrated Fromberg's valiant effort to gain the biggest victory of his career, narrowly defeating him in the first match of the 1990 Davis Cup final round, 4-6, 6-2, 4-6, 6-2, 6-4.

"Coming into the match I anticipated winning it a lot easier than I did," Agassi said.

In the other match of the evening, American Michael Chang, 18, defeated Darren Cahill, 6-2, 7-6 (7-4), 6-0.

The victories in front of 17,994 fans at the Florida Suncoast Dome gave the United States a 2-0 lead over Australia in the five-match tournament. The heavily favored U.S. team hopes to win its first Davis Cup since 1982. A victory in the doubles Saturday would guarantee that. That the Agassi-Fromberg match went five sets was in itself a minor victory for the underdog Australians. Ranked at 32 in the world, Fromberg was given little chance of beating America's best player.

Agassi, 20, blamed his somewhat lackluster performance on a virus and said he had not been sure whether he would be able to play.

"I didn't eat for three days," Agassi said. "I was lying on my back."

He said he told U.S. team captain Tom Gorman to have Aaron Krickstein ready to substitute. "There is no question that to pull out a match like that . . . is something to be proud of," Agassi said. "No way I won this on my tennis alone. I had to dig a little deep to dig this one out."

Fromberg had beaten Agassi in their only other meeting, a three-set match on hard courts in Cincinnati last summer, and that obviously gave him confidence. In fact, he was somewhat annoyed that Agassi used illness as an explanation for the close score tonight. "It's alot of rubbish, really," normally shy Fromberg said. "He played the best he could play . . . I feel it was a very close match and it could have gone either way."

Fromberg's ability to compete so well on clay was not entirely a surprise. He had played 29 matches on clay this year, winning 19, including his tournament victories in Italy and Switzerland.

Those experiences clearly figured into Australian team captain Neale Fraser's decision to use Fromberg rather than more powerful and more experienced Pat Cash for singles this weekend.

Fromberg, who is 6 feet 3 and weighs 168 pounds, uses a two-handed backhand, but otherwise is a staid contrast to Agassi. He showed grit but little flash, other than an occasional cannonball forehand and 15 booming aces.

Agassi came to Florida after completing his best year ever. Ranked number four in the world, he was a finalist at the French and U.S. opens. He also posted two impressive victories over Germany's Boris Becker and one over top-ranked Stefan Edberg of Sweden.

And this month he won the ATP championships in Germany, one of two year-end tournaments reserved for the top men in the world.

Tonight's three-hour 36-minute match was a typical clay court slugfest, with long rallies from the baseline, and an uncharacteristic number of unforced errors by both players. Fromberg showed unusual patience for a player with so little seasoning in big matches. In the first set, he surged ahead to a 5-2 lead, using a mixture of power and deep, arching shots that Agassi could return only at shoulder height. "I was a little bit nervous, but I felt I had nothing to lose," Fromberg said.

After cruising through the second set, Agassi made matters difficult for himself. He seemed to lose concentration, playing the big points without any verve. He came alive again in the fourth set, attacking with more authority and putting Fromberg on the defensive. Although Agassi had plenty of chances to polish off Fromberg in the fifth set, the end was a seesaw. Agassi finally won on his third match point.

It was only the second time in seven tries that Agassi has won a five-set match.

In the second match, diminutive Chang had little trouble disposing of Cahill, a doubles specialist ranked 57th in the world.

Although Chang was injured with a stress fracture in his hip early this year and had won only one tournament since recuperating, his superior consistency and patience proved too much for 25-year-old Cahill.

In addition, 15th-ranked Chang prefers the slow clay surface; Cahill had played only two matches on clay this year.

Agassi, facing a possible suspension for withdrawing from next month's Grand Slam Cup in Munich, said later he has reversed his decision and will play in the $6 million tournament. He said he notified the ITF Thursday and will donate any winnings for the event to charity. He had pulled out in support of the ATP Tour.