You can remember the meaning of 回合 with this easy mnemonic:
After a knee injury the past boxing champion has returned (回) to the ring to meet his rival once again first round of the day. This time with vengence for the wilful injury the other inflicted on him. The champ wishes to close (合) the past and start a new chapter in his career.
Using mnemonics to remember the meaning of Chinese characters like can have an enormous impact on how long to learn Mandarin it takes.