As mentioned earlier, Mardy Fish surprised fans by taking the first set from Roger Federer in a tiebreaker. Turnabout being fair play, Federer returned the favor and won the second set via tiebreaker after Fish forced the issue.
Fatigue may play a part in the rest of the match; as the CBS crew pointed out, Fish has spent almost seven more hours on the court this week than Federer. Further, the temperature and humidity are climbing in Mason--the heat index is up to 89 degrees--which would only exacerbate any existing differences in conditioning at this point in the tournament.