I came across the following sentence
If A team is the winning team...
Why is the に particle being used in
Why would it not be
Aチームは in this case?
I have not come across an explanation on this usage of
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First, your translation of the example is not correct, but that doesn't matter with respect to what you want to know. The example means "if there is a team that beats Team A, then...".
The verb 勝つ has the valency が―に, whereby が marks the subject, and に the object:
subjectが objectに 勝つ
The antonym of 勝つ is 負ける, which also has a が―に valency.
One famous explanation of verbs like these can be found Shigeru Miyagawa's Structure and Case Marking in Japanese (Syntax and Semantics vol. 22).