Venus Williams, the 14-year-old tennis debutante who made a triumphant transition into professionalism with a straight-set victory on Monday, had the welcome mat abruptly yanked from beneath her tonight by Arantxa Sanchez Vicario, the world's second-ranked player.

But Williams did not submit to the Spaniard's greater authority without a stirring fight. Instead, she put on a bravura show in the opening set, and even had the veteran down and dazed in the second before her youthful enthusiasm evaporated into a 2-6, 6-3, 6-0 comeback by Sanchez Vicario.

This was an auspicious debut by a prodigy who had not played a tournament match since she was a 10-year-old juggernaut on the local kiddie circuit. Asked how this second-round loss compared with previous defeats, Williams shook her braided head in bemusement: she had never, she enlightened her questioner, before lost a match.

And she wasn't distraught about this one.

"I wasn't going to go out there and let her hurt me," Williams said of Sanchez Vicario, whose 1994 record includes two Grand Slam crowns, three victories against top-ranked Steffi Graf, and more than $2 million in earnings. "I expected to play better, but I guess she knew how to come back and knew what to do.

"But I had nothing to lose, nothing to win: I was just having fun," said the teen-ager, who grabbed a microphone and thanked the crowd and the Bank of the West Classic's sponsors for making her first tournament "so much fun."

Each player received an accolade upon taking the court: Williams, who is turning out to be an introvert who doesn't mind the attention being showered on her extroverted tennis game, beamed through her introduction.

Then came her first test from a champion, and through the first three games, she passed it so flawlessly she assumed a 3-1 lead.

"I want to make a statement," Sanchez Vicario said later, joking. "I think 14-year-olds should not play professional tennis. Seriously, in the first set I was watching more the way she hit the ball instead of thinking about my shots."

Standing well inside the baseline to get an early pounce on Sanchez Vicario's serve, Williams took a 2-0 lead by cracking a backhand return crisply down the sideline. When the Spaniard tried to trick the rookie with her patented drop shot, Williams used the long legs that have quickly become the envy of every player here to gallop to the net and earn her own winner.

Aware of her opponent's reputation as the consummate counterpuncher, the teen-ager assumed a simple strategy: she made it her business to deliver outright winners and not supply the opponent with anything to counterpunch.

Sanchez Vicario, down 3-0 and looking rusty after a layoff that began the day after her United States Open victory, halted the ingenue's momentum only briefly. She held serve to claim her fourth game, and broke the lavender-skirted Williams in the fifth to close the gap to 3-2, but that's as close as she got.

Williams broke back for a 4-2 lead when the Spaniard blooped a forehand long and held her serve at love for 5-2. Then, with a little extra spring in her step, she claimed the set with yet another break when Sanchez Vicario overhit a backhand at double set point.

The teen-ager continued to pour on the pressure, and take some giddy risks, in the second set. She got off to a 1-0 lead by pelting an ace past her flummoxed opponent at game point.

Sanchez Vicario survived a pair of break points against her in the second game, but in the fourth, Williams closed in and broke her again. Trailing by 15-40, the Spaniard saved one break point with an angled overhead smash to the corner. But on the next, Williams hustled into net and defied another drop shot by guiding a game-winning backhand down the sideline for a 3-1 lead.

Sanchez Vicario, correctly sensing it was time to get her own game into overdrive, did just that. In fact, she composed herself again so completely that she shut Williams out of the remainder of the match.

"At 3-1 I thought she was a little tired, and I knew that was the moment to start playing my game if I wanted to win," said the victor.

While Williams steadily ran out of adrenaline, the durable Sanchez Vicario started to flaunt hers: she ran the teen-ager into distant corners, took full advantage of the flagging pace of the ingenue's serve, and used her experience to deflate the brash but increasingly bewildered challenger.

"Finally we've got a tool to use, a barometer," said Rick Macci, Williams's coach, who was hoping the match would prove an exhilarating object lesson for his pupil.

According to Richard Williams, who predicted that this would be a three-set struggle that would end in defeat for his daughter, the loss effectively ended his daughter's professional commitments for the remainder of the year. He said the family would return home to Florida and that Venus, who will continue to train at Macci's Delray Beach tennis academy, will not play more than five matches in 1995: for now, tennis is very much a part-time job.

Williams also insisted that he was in no hurry to pursue any of the cavalcade of business opportunities that have been presented to his daughter.

"I want her to be a little girl," he said, "and you can't do that when you've got a dozen corporate sponsorships to answer to."