So his not-surprising decision to miss it leaves a hole and none of the top contenders, from a tennis point of view, would be too upset to see his name missing from the draw.
On summer form, this is a wide-open event. Top seed Novak Djokovic was sort of a newlywed ghost during his two rather brief appearances in tuneup events in Toronto and Cincinnati. No. 3 seed Stan Wawrinka still seems to be suffering some late Australian Open reverb, because he's not anywhere close to the player he was in January when he became the first outside the "top group" to pull off a major.
Canadian Milos Raonic also is in good form despite a lopsided loss to Federer in Cincinnati. He won the ATP 500 event in Washington, D.C., reached the quarter-finals in Toronto and the semi-finals in Cincinnati. That was good enough to win the "U.S. Open Series" crown. And he does not have Federer, against whom he is now 0-6, in his half.
Here are the paths to the final four for some of the contenders; we grade them from A to D (A being the relatively easier draw, D being a tough one).
Paths to the final four
 Novak Djokovic
Diego Schwartzman (ARG) -----> Gilles Muller (LUX) ----->  Garcia-Lopez / Querrey ----->  John Isner ----->  Murray /  Tsonga ----->  Wawrinka /  Raonic.
Draw grade: C-. Potentially three big servers in a row in Muller, Querrey, Isner. Then possibly the winner of Murray and Tsonga. Big challenge.
 Roger Federer (SUI)
Marinko Matosevic (AUS) -----> Sam Groth (AUS) ----->  Ivo Karlovic (CRO) ----->  Fognini /  Bautista-Agut ----->  Dimitrov /  Monfils ----->  Ferrer /  Berdych
Draw grade: A. We're not handing him the trophy just yet. But no Djokovic in his half, no Tsonga, no Murray, and a potential quarter-final opponent against him he has a crazy-good record either way. And we know he can handle the big servers. It's all there for him.
 Stan Wawrinka (SUI)
Jiri Vesely (CZE) -----> Nicolas Mahut (FRA) ----->  Jérémy Chardy (FRA) ----->  Robredo /  Youzhny (or Kyrgios or Pospisil) ----->  Milos Raonic (CAN) ----->  Novak Djokovic (SRB)
Draw grade: B. It doesn't seem quite as potentially routine as Federer's draw, and it can't ever be an "A" simply because Wawrinka isn't playing that well. But it's not bad.
 Milos Raonic (CAN)
Qualifier -----> Benjamin Becker (GER) ----->  Lukas Rosol (CZE) ----->  Kei Nishikori (JPN) ----->  Stan Wawrinka ----->  Novak Djokovic (SRB)
Draw grade: B+. A healthy Nishikori – and that's a question mark – is the type of player who could give him fits in the round of 16.
 Andy Murray (GBR)
Robin Haase (NED) -----> Radek Stepanek (CZE) ----->  Fernando Verdasco (ESP) ----->  Jo-Wilfried Tsonga (FRA) ----->  Novak Djokovic (SRB) ----->  Wawrinka /  Raonic
Draw grade: D. Worse than Djokovic's, with some talented, tricky customers in every single round. Just about the worst-case scenario.
 Jo-Wilfried Tsonga (FRA)
Juan Mónaco (ARG) -----> Aleksandr Nedovyesov (KAZ) ----->  Julien Benneteau (FRA) ----->  Andy Murray (GBR) ----->  Novak Djokovic (SRB) ----->  Wawrinka /  Raonic.
Draw grade: B. Tsonga looks good until the matchup with Murray which, on current form, has to have him as the favorite.
First-round matches to watch
 Jo-Wilfried Tsonga (FRA) vs. Juan Mónaco (ARG): One for the ladies; no doubt some of the gents, too.
 Julien Benneteau (FRA) vs. Benoît Paire (FRA): The great thing about the French players is that they're all so ... different from one another. There's no cookie-cutter at the French Tennis Federation. And this is one of those "opposites attract" matchups between the workmanlike Benneteau and the mercurial Paire, a matchup in which the backhands will trump the forehands.
 Tomas Berdych (CZE) vs. Lleyton Hewitt (AUS): Every Grand Slam Hewitt plays could conceivable be his last time through that city, although he's given no indication he's wrapping it up. Berdych isn't in peak form; this one could be a battle.
 Andy Murray (GBR) vs. Robin Haase (NED): Haase has never lived up to his promise, really. But the last time he played Murray at the U.S. Open, in the second round in 2011, Murray had to come back from an 0-2 set deficit to win in five.
 Mikhail Youzhny (RUS) vs. Nick Kyrgios (AUS): Youzhny, who seems to be winding it down, has shown only occasional signs of life this year. Kyrgios, the 19-year-old Wimbedon sensation (he upset Rafael Nadal there), went back home to Australia for a break after Wimbledon. He lost in the second round in Toronto to Murray, and pulled out of the Cincinnati qualifying because of "bone stress in his left arm." He's probably fine, but he's short on matches, it's best-of-five sets, and Youzhny has a lot of experience.
Bernard Tomic (AUS) vs. Dustin Brown (GER): A terrific clash of game styles and on-court personalities here. Call it "Dreddy vs. Bernie".