He is saying "Allez" here but he says "Come on" more than "Allez" i think. I wonder why. His mother language is french.
no, he spoke Swiss German and English at home (because his mum is from South Africa). didn't really start having to speak French until he went to live at the tennis academy in the French part of Switzerland as a teen.
I speak good French and bad Spanish, but voted "come on." Come on, as it is used in English and on the tennis court, almost directly calls out the opponent imo (and can serve to thoroughly piss me off when said after an error by me). Allez and vamos are more like saying "let's go" in English, which I consider to be more of a self pump-up than a calling out of an opponent.
Ajde is goofy, and Serbia doesn't have many people or popular understanding from the greater world (no offense to our Serbian friends, as they produce some fine athletic talent for such a small country), so that would have no effect on most. Screaming "YAAA" might work, I dunno.
I don't know if it's goofy, but I don't recall it being used as a rallying cry. Mostly I associate it with "hurry up" (as my mum saying it to me when I was taking forever to get up in the morning for school