Jimmy Arias, 16 years old, is growing up fast. Last year, when he was the youngest player on the men's tour, Arias weighed only 115 pounds and didn't have enough strength to compete seriously with grown men twice his size and age.

But Jim Arias (Jim sounds older than Jimmy) is adult now. He is growing a mustache. "I travel all by myself now," he said yesterday. "I even went to Italy by myself this spring."

The biggest measure of Arias' maturity is his improvement on the court. He now weighs 135 pounds and wallops the ball like a light heavyweight. Slashing topspin forehands and serving to spots like a young veteran, Arias defeated Bill Scanlon, the 16th seed in the Washington Star International, on a side court yesterday, 6-2, 6-4.

The youngster is becoming a crowd favorite at the Rock Creek Park tennis facility.

"I figured I had nothing to lose," Arias said afterward. "Scanlon had all the pressure on him. He was seeded and ranked and playing against a 16-year-old. I'm pretty tournament tough now. I've played five straight weeks now and I'm used to the pressure."

Arias was ranked about No. 500 when he lost his first-round match to Jaime Fillol on the Stadium Court last season. He's currently No. 168 and says, "I should be in the top 100 when the next rankings come out."

Arias used to get frustrated when girls his age playing the women's tour reached the top five in the world -- Tracy Austin, Andrea Jaeger, Pam Shriver. "But now I realize it's very tough for a 16-year-old male to do well against these men," he said.

Arias, who recently finished 11th grade in Miami ("I'm no dummy"), helped his game by taking six months off the tour to work on his physical conditioning. "I took six months of Nautilus, three times a week," he said. The improvement is dramatic. Muscles have formed in his arms and legs.

The shots he hits now are powered by muscle, not looped over the net. He was trailing Scanlon, 4-3, in the second set, but broke serve and won the next three games to take the match and receive a nice ovation from the crowd.

He plays Czech Pavel Slozil or Bolivia's Mario Martinez in his next match and has a good chance to advance another round.