Tennis, like pool, is often self-refereed, but in bigger events, there is an official umpire/ref.
Here are some examples of good sportsmanship in tennis matches, from
http://www.ethics.org/resources/article_detail.cfm?ID=819 , where the players had no "duty" to do wha tthey did, but they still do because they value their integrity. The article has many examples, but I'll highlight a couple involving big names in major events.
... he cites a match he watched at the French Open a few years ago, when two Spaniards who were also friends faced each other in the finals. Neither had ever won a Grand Slam tournament. Carlos Moya won the first set and had broken Alex Corretja's serve once in the second. He was serving, however, and was behind in that game. Moya hit a powerful serve that was called out. If the call stood, it would have given Corretja the game and evened up the score. But Corretja overruled the linesman, giving the crucial point to Moya. The game went back to deuce and Moya eventually won.
In another case, Andre Agassi was playing in a major tournament and was scheduled to meet Pete Sampras in the final. Sampras, however, came down with food poisoning and could not get himself together before the time of the match. Agassi was told he had won by default. Instead, he suggested waiting to see if Sampras would feel good enough to play a little later. He did, they played the match, and Sampras won. This experience confirmed Steve's belief that both are great sportsmen and exemplary champions.
In each case the player makes a decision that is against the official ruling and that goes against their own self interest. Lots of people in pool would act the same way. Lots of people in tennis wouldn't.
Many posts in this thread point out that it is not the players' responsibility to make calls against themselves, and that it is not cheating not to make a call against yourself that overrules an official's call (or lack of call). Be that as it may, people still do it all the time, not because they are obliged to, but because fairplay and sportsmanship means more to them than winning at all cost. Some people seem to think that's stupid.