Can big four come back with gusto?

You could see this train wreck coming a few years ago when they put together the compressed 2012 schedule.

Today's professional tennis circuit is tough enough on the human body, but when you shoe-horn another important event -- London 2012 -- into the middle of the calendar, you are asking for chaos.

And that's just what we have among the elite players two weeks before the start of the U.S. Open.

The two North American tournaments in August, Montreal/Toronto and Cincinnati, are usually star-loaded, competitive and good indications of where the top players are going into New York. This year, in the wake of the Olympics, where many of the best went deep, it's tough to make any kind of meaningful assessment.

Let's review the list of predictable attrition:

• No. 3-ranked Rafael Nadal, the defending gold medalist from Beijing, skipped the Olympics and will now miss both North American events, each for the first time in his career. Chronic tendinitis in his knees is the culprit. If he makes it to the line in New York, it will be two months between matches for Rafa, going back to his second-round loss to Lukas Rosol at Wimbledon.

• No. 4 Andy Murray, the freshly minted singles gold medalist, won his first match in Toronto, beating Flavio Cipolla 6-1, 6-3, then pulled out of his second citing a left knee injury he tweaked running down a forehand. No. 1 Roger Federer, who lost to Murray in a straight-sets Olympic final, did not play in Toronto. Neither did No. 5 David Ferrer.

• No. 6 Jo-Wilfried Tsonga and No. 8 Juan Martin del Potro actually showed up in Canada, but -- some might say predictably -- lost their first matches, to Jeremy Chardy and Radek Stepanek, respectively. No. 7 Tomas Berdych went out to Richard Gasquet in his second match.

• On the women's side, Wimbledon and Olympic champion Serena Williams passed on Montreal, as did No. 2-ranked Maria Sharapova, citing a "bad" stomach virus she picked up in London. Sharapova, who won only one game against Williams in the Olympic final, also pulled out of Cincinnati.

• No. 1-ranked Victoria Azarenka had her first match against Tamira Paszek suspended by rain at 3-all last Thursday night. On Friday, faced with the prospect of two matches in a day, Azarenka withdrew citing, yes, a knee injury.

"How much gas is in the tank?" asked Justin Gimelstob, who was an Olympics analyst for NBC. "That's the question going into the U.S. Open. The thing you've got to remember about New York is that it's a back-loaded schedule. By playing first-round matches over the first three days, it means late in the second week you've got to beat three of the best players in the world in a span of five days. That's a huge physical ask."

Give credit, though, to the players who made the effort to play in Canada. Novak Djokovic showed some grit, working his way to the title in Toronto. Sharapova, meanwhile, flew to Montreal and attempted to practice before withdrawing.

"It's part of the game, unfortunately," said Rogers Cup tournament director Eugene Lapierre. "Maria showed exceptional professionalism. We wish Maria a quick recovery and hope fans will see her shine soon."

Beyond Sharapova and Nadal, you'll probably see most of the above playing in Cincinnati with customary gusto. You can't blame players for trying to create some downtime for themselves. It's a professional necessity. Even Ferrer, who is usually a glutton for punishment -- the Spaniard has averaged 77 matches over the past three years -- decided not to play with his match count already at 63.

Federer, who turned 31 last week, has learned to curb his match-playing enthusiasm, and it has paid off. After losing to Andy Roddick in Miami, Federer was off for more than five weeks before meeting Milos Raonic in Madrid. That was one reason Federer was able to win his 17th major at Wimbledon. Like Djokovic and Murray, he did not play any tournaments between Wimbledon and the Olympics.

Nadal, more than any player, remains a long-term question.

"Rafa," said Gimelstob, also a Tennis Channel analyst, "is a huge enigma. I was shocked when he pulled out of Cincinnati. I thought he'd hibernate for a few weeks and then come to Toronto, come out firing and catch the other guys off guard.

"He's obviously got serious issues with the knee. The guy loves to play, so he's probably bouncing off the wall. At the moment, he's a variable impossible to assess."