American University put on an impressive display of fast-break basketball tonight, racing past a tired Rider team, 99-80, in an East Coast Conference tournament semifinal at the Palestra.

The Eagles (24-4) will play St. Joseph's (21-7) Saturday at 9 p.m. for the championship and the automatic bid to the NCAA playoffs that goes with it. St. Joseph's easily defeated Lafayette, 73-63, in the other semifinal.

Rider had not won in the Palestra in its previous 17 games and AU jumped ahead 6-0, before the Broncs scored. Five quick points by Derrick Miller pushed Rider ahead, 7-6. It was the Broncs' last lead.

Three points by Ed Sloane, AU's leading scorer with 22 points, and the first of four first-half rainbow jump shots by Robin Hoey put AU in front, 11-9. Rider tied the game seconds later before the Eagles' running game got in gear. Five other Eagles also scored at least 10 points.

Hoey's long jump shot started an 8-0 blitz and the Broncs may just as well have started back to Lawrenceville, N.J.

"We wanted to play an up-tempo game," said AU Coach Gary Williams, voted the conference coach of the year for a season in which the Eagle have set a record for victories and have an outside shot at an at-large NCAA bid even if they don't win Saturday night.

"I wasn't happy with the way we played against Drexel (a 62-60 quarterfinal victory Wednesday)," Williams said. "We were sluggish on offense, mainly because we played poor defense."

Williams had no complaints tonight. Sloane, Hoey, Mark Nickens and Dennis Ross did a fine job of stopping the Rider offense before it got started in addition to keeping the aggressive Broncs off the boards most of the first half.

The AU shooters also scored repeatedly over Rider's slow-moving 1-3-1 zone until AU had forged ahead, 33-21, with six minutes to play.

"There were no brakes on tonight," said Sloane. "We wanted to get on top and stay there. We didn't let Rider create the action the way we allowed Drexel to do. We forced the play."

Had it not been for forward Kim Carter (12 points), a 6-foot-6, 200-pound imitation of a brick wall, Rider might have faded out of sight in the first half. Carter came off the bench and muscled in six layups, three of them coming in a row to cut the AU lead to five, 36-31.

But the Eagles, getting inside the picking up fouls, sank seven free throws in the waning minutes of the half to gain a 49-37 halftime lead.

Rider tried its best to make a game of it in the second half but the Broncs lacked stamina and the rally fizzled. Four baskets by Miller cut the AU advantage to five points. Rider would get no closer.

"After the Drexel game, there was no way we would let down tonight," said Hoey. "The difference was our bench. They came in and did a good job. Our press wore Rider down and when they lost point guard Fred Lee, they couldn't run with us."

If Rider had three Fred Lees, it couldn't have kept pace with the Eagles. Nickens, who was superb with 15 points on six-for-six field-goal shooting, four rebounds, eight assists and three steals, and Dennis Ross, who had 14 points and six rebounds, made sure the Broncs didn't get too close. Those two and Sloane led another AU surge that sent the regular-season ECC champions roaring out to a 79-62 lead, with 6:46 remaining in the game.

"That was it," Williams said. "I told the guys before the game, we were going to run 40 minutes. And we did. We tried to make our defense force us to play better offense."

The final shapes up as a revenge game for St. Joseph's, which dropped an 84-83 overtime game to the Eagles two weeks ago. St. Joe's front court, burned by AU quickness in that game, was dominant tonight against Lafayette. Forward Bob Williams, a nonfactor in the last AU game, had 22 points and nine rebounds; forward John Smith, who fouled out in regulation against the Eagles, added 16 points and 11 rebounds.