I believe he would have had a much better chance against Mr. Nadal during the heat of the day when the balls were clearly sailing through the court faster. This is probably why he requested day matches, esp. with Del Potro.
it's not only a matter of day or night, it's also a matter of heat :
- during the day, when the weather is hot, balls fly and turn in Australia, which is a problem for attacking players to guess exactly where the ball arrives to take it precisely : Fed has described it in the past, Fish and Tsonga also described it this year (Fish said that trajectories were not "straight" contrary to the US Open conditions hence why it's easier for an attacking player in the US Open according to him, not because it's quicker) (see also the poor quality of Nadal-Lopez match or Fed-Wawrinka last year). There's also a matter of light : Fed clearly prefers the light of the night sessions and especially dislikes the moment when there's half a shadow on the court. Remember Davydenko's match and remember also Del Potro's match : when the shadow disappeared in the beginning of third set, Fed played much better, and in his post-match interview Fed talked again about how hard it was when there was half-a-shadow on the court in the second set.
- during the day when the weather is cold, the ball is easier to control and the balls are quite quicker than in the night
- during the night, when the weather is cold, the balls are heavy and the conditions are slow, see Fed's match against Nadal this year and against Simon last year (it was even colder)
- during the night, when the weather is hot conditions are ideal for Federer : quick conditions, balls with straight trajectories, easy to control, it's always the conditins where he played his best matches. If Hewitt's coach said that, it's maybe because he remembers a perfect match Fed had played against Hewitt at night during a hot day two years ago.
Then you see :
- when the weather is hot (nearly all fortnight, only one day was cool : against Nadal's day, this tournament is like a sign of God for Nadal ), it's better for an attacking player like Fed to play at night
- whne the weather is cold, it's better for a player like Fed to play during the day.
As for Fed-Del Potro's match (which was a hot day), some people said that "Fed had asked a day-match", I have two theories about that :
- he didn't really ask that : it was normal that Nadal had the night-match as Fed had had it previous round, maybe he just said OK, maybe he just asked to start at 4.30 for the men's match, which was not usual start (Murray-Nishikori started at 2.30)
- maybe he asked it ... but not for him : for Berdych, as for him if he wanted to win the tournament of course it was much more likely if Berdych beat Nadal ... and for Berdych in a hot day, a night match was much better, during the day the balls would have been difficult to control and also Berdych would have been quickly exhausted with his ballbashing style.