More delusional than "other people".
Don't think so. Tennis players are those "other people" who just happen to be playing tennis.
Other people are delusional about many things- their job, their religon, their marriage, their kids, their political party, football teams, dieting, appearance, weddings, intelligence, parents, clothing, fast-foods, cars-- on and on--
So, why should they be less delusional when it comes to a sport that they may associate strongly with their own self image?
Is a tennis player more delusional than (for example) a student of Zen- or just delusional in different ways?