Tracy McGrady, the U.S. leading scorer after three games in the Olympic Qualifying Tournament, sat out the second half with tightness in his back after scoring 10 points. It is not believed serious . . . The U.S. had 38 assists on 42 baskets . . . The U.S. is off today and will start second round play tomorrow, facing the four advancing teams from Group A (Argentina, Mexico, Canada and Puerto Rico). The semifinals are Saturday with the finals Sunday.