Madison and Potomac (Md.) defeated previously unbeaten teams yesterday at Georgetown University's McDonough Gymnasium and tonight will play in the boys championship game of the second Washington Post Holiday Classic.

No. 7 Madison used a big third quarter to pull away and defeat No. 16 Chantilly, 69-54, and No. 14 Potomac handed No. 17 Good Counsel its first loss, 77-67, in semifinal games.

Madison and Potomac meet in tonight's championship game at 8:45.

Potomac (5-1) opened its game with its patented full-court press and quickly ran away from the Falcons (9-1). The Braves finished the first quarter with 29-13 edge.

Guard Robert Garner (26 points) made a three-point play and two three-point shots in succession as Potomac moved to an 18-8 lead. The Braves also controlled the rebounding in the quarter and Patrick Evans scored on two offensive rebounds. Garner and Ellis Jones also had rebound baskets.

"We rely on our defense real hard and we just wanted to come out and go after them," Garner said.

Potomac maintained a 15-point halftime lead, but the Falcons made it interesting in the third quarter with a press of their own. They forced four consecutive turnovers and pulled to 47-39. Later in the quarter Good Counsel trailed by seven but Garner's three-pointer with four seconds remaining in the period pushed the Braves' lead back to 10.

Behind the outstanding play of guard Kurt Small (tournament-record 36 points) the Falcons stayed within striking distance in the fourth quarter but could get no closer than six as Potomac made 10 of 12 free throws in the final 2:07.

The first game pitted Great Falls District rivals and unbeaten squads. With Omar Illidge shooting nine for nine from the field and Bart Lammersen scoring 20 points, the Warhawks (6-0) prevailed in what could be the first of six meetings between the schools.

But it was defense that enabled Madison to beat Chantilly (5-1). Scott Wenk was given the task of guarding the Chargers' top scorer, Shane Clancy. And Wenk did more than anyone could have expected.

Clancy didn't score until 35 seconds remained in the third quarter; that basket accounted for his only points. At that point the Warhawks held a 51-37 lead.

"Their guards are solid all-around players and we wanted to focus on them," said Wenk, who scored nine points. "Clancy is tough to shut down once he gets going and their screens are the hardest ones we've seen. But we knew a little about them and we knew where the screens would be."

Chantilly struggled offensely, and in one nightmarish stretch, comitted seven turnovers in 10 possessions as Madison built a 49-31 lead.