Mike is a great football player. last saturday, his new girlfriend attended the game to watch him play, and mike played even better than usual. mike's enhanced performance demonstrates the phenomenon known as

Mike demonstrates the phenomenon known as social facilitation.
This phenomenon is also known as the audience effect. This means that the presence of an audience is likely to cause a particular effect on a performer. In this case, Mike was positively influenced by the presence of his girlfriend, which is why his performance was enhanced as he didn't want to disappoint her.