"To reach the semifinals" means to progress to the stage of a competition where there are only four remaining teams or individuals remaining, competing for a chance to move on to the finals. Full definition
Darlene Hard, still a junior in tennis ranks, startled experts by reaching the semifinals of the nationals at Forest Hills before losing to Louise Brough, 1947 national singles champion. (si.com)
Tennis player Gene Scott wrote of the miracle of reaching the semifinals at Forest Hills. (si.com)
No U.S. player had gone as far in the tournament since Miles reached the semifinals in 1959. (si.com)