Complex sentence: The two finalists had met earlier in the competition, during the pool stage, when South Africa won 36--0, so South Africa began the final as the only undefeated team left in the competition.
Split into simpler sentences: The two finalists met earlier in the competition during the pool stage, with South Africa winning 36--0. As such, before the final, South Africa were the only undefeated team left in the competition.