Throughout the long history of the US Open there have been countless upsets, going all the way back to the beginning of the tournament in 1881. The more recognizable upsets have occurred in modern day, at least for the casual fan.
Since the USTA switched over from the clay surface to hard courts in 1978, the athleticism and quality of play year in a year out has vastly improved. The underdog stories are always well covered, and let's be realistic, everyone does love an underdog. In addition, not all underdog victories significance is based on the round they occur, but rather they are decided by ranking differential and overall experience/careers.