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What about having tennis bets? I'd personally bet in them, at least having them for important events like Rolland Garros, Wimbledon, Madrid Mutua Open, US Open, Australian Open, etc.
but as you said, maybe they see the bets and know a good player and feel like betting, almost nobody would bet against Djokovic, Federer or Nadal in the first rounds
Only possibly finals, adding big matches won't have much interest unless there is something to play for. What you've got to remember is 95% of the forum don't have a care cup to fill about Federer or Nadal but if there is a major trophy at stake only around 85% won't have a care cup.