It's the definitely the most important to date and here's why:
If Mexico defeats Venezuela on Monday afternoon and Puerto Rico loses to the Dominican Republic in the nightcap, Mexico would finish with a tournament-best 4-2 record and win the title, its first since 2005.
If Puerto Rico wins and Mexico loses, Puerto Rico would finish with a 4-2 record to win its first Caribbean Series in 11 years.
"There are going to be two teams happy or sad, and that's baseball," said Mexico manager Eddie Diaz.
If Mexico and Puerto Rico win, both would finish with a 4-2 record and would play an extra game Tuesday at 9 a.m. ET to determine the tournament winner.
However, this isn't just a two-team race. The Dominican Republic squad still has a chance to defend its 2010 Caribbean Series title, and 2009 champion Venezuela is alive as well.
If Mexico and Puerto Rico lose, there would be a quadruple tie at 3-3, in which case the two teams with best run differential would play a tie-breaker on Tuesday.
Entering the final day of play, Puerto Rico owns the best run differential at +4, while Mexico is at +3, Venezuela at -2 and the Dominicans at -5.
"We put ourselves in a bad situation," Dominican Republic manager Dean Treanor said after Sunday's 3-0 loss to Venezuela. "I didn't think we would play so bad."