upon normally means physically on top of something, but in this sentence it means the same thing as the phrase "Once upon a dream". upon here means something along the lines of "Within the instance of." The sentence you have provided is saying that within the brief moment you enter the room (before you have a chance to really analyze your surroundings) a visitor (somebody not already familiar with the lounge) might think that the tournament was about to take place within the lounge. I dont know the full context but I'm going to assume that the lounge was set up for a party to view said tournament on something like a television, and the room had been decorated very well to fit the theme. Hope this helps :)