"There are days," Roscoe Tanner said, "Where nothing goes wrong and it almost scares you because you're almost not in control. You just know it ain't gonna' last very long."

It was one of those kind of days for Tanner yesterday in the quarterfinals of the Volvo Classic tennis tournament. He needed only 15 minutes to blitz Brian Teacher, 6-0, in the first set and was leading, 3-0, in the second.

Teacher rallied smartly to take that second set, 7-5. But that only made Tanner try harder in the third. The shots that came so easily for him in the first set returned in the third, and Tanner, the fifth seed here, prevailed, 6-1, for the match and an appearance in the semifinals Saturday.

Tanner advanced to a semifinal confrontation with fourth-seeded Raul Ramirez, who needed only 54 minutes to dispose of Tim Gullikson, 6-2, 6-2, last night.

"In that first set, I just couldn't miss," said tanner, "but there aren't too many sets or days you'll have like that. When I play well like that, I have a tendency to go for more and more and usually I get less and less."

In that first set, all Tanner could do was shake his head, sigh heavily and wonder if Tanner would ever miss, nearly every shot he hit came rocketing past him for a winner. "I'm flat," Teacher said to a friend in the stands, and for a while, it looked as if a steamroller was doing the damage.

Tanner's rocket first serve left Teacher handcuffed, and most of the time when he came to the net, Tanner passed him with a variety of shots.

When Teacher finally held service in the fourth game of the second set for his first game of the match, he waved his finger in the air in celebration.

"A lot of times when you're losing that badly," Tanner said, "when you do finally win a game you feel like a cloud has lifted off your head. The thing I didn't do was bear down on my serve at 4-2. I wasn't concentrating.

Teacher suddenly found his strokes, particularly a brilliant running passing forehand shot down the line that won several key points in the second set.

Teacher broke Tanner in the seventh game on a backhand passing shot down the line. Both men held service until the 11th game when Teacher hit a fine volley at the net at deuce and won the game when he hit a backhand down the ine on Tanner's first serve for a 6-5 lead.

Tanner had a break point in the next game, but Teacher put away three straight volleys to take the set, 7-5.

"At that point, when I lost the second set, I said to myself 'you've got to bear down and hit the ball,'" Tanner said. "He had started running better, he was doing a lot of good things. I just felt it was even, and I had to reestablish myself in the match."

And so he did.

Teacher hurt himself badly in the volley. Leading, 5-1, Tanner won the double faulted after landing, 40-30. Teacher missed a volley to give Tanner the advantage, then doublefaulted again to give him the game and a critical break.

Tanner broke him again in the fourth game, winning the game on a crisp forehand Teacher netted on the volley. Leading, 5-1, Tanner won the set and match with four straight points, all of them a result of Teacher errors returning serve.

"What I really liked about this," Tanners said, "was that I played well, I went off, and I was able to pick it up again. In the past, if I went off, I might not come back."