An example of gamesmanship I often see is the server who only calls the score out if they are ahead. They would never call the score out if they're down 15-40, but when they're up 40-15 they always call it out loud and clear. Sometimes I'll retaliate by calling the score for them--in the middle of their service motion.
Another gamesmanship thing they'll do is journey to the far reaches of the court to gather up a ball that is not needed. They'll do it to try to throw off your rhythm. Sometimes they're old guys who need to catch their breath. I don't mind it so much if it's of necessity, I don't think they are even conscious of it sometimes.
"...the human emotional system was not designed to endure the mental rigors of a tennis match." Dr. Allen Fox