Wawrinka, Sharapova injuries more proof no one is safe this summer

The sultry days of late summer are laid-back, but in tennis the operative concept is laid-up. That's just part of the reason this hard-court season could be the most unpredictable in recent memory.

Stan Wawrinka, the defending US Open champion, announced Friday that he's finished for the year because of a knee problem.

"After sitting together with my team and considering all options, I have made the very difficult decision to end my 2017 season," the No. 4-ranked Wawrinka told the ATP media department. "I need to undergo medical intervention on my knee."

The once utterly dominant Big Four, plus Wawrinka, has been whittled down to a big two, Rafael Nadal and Roger Federer.

Novak Djokovic lit the fuse on this issue when he retired during his quarterfinal match at Wimbledon last month, then declared that he's done for 2017 because of nagging pain in his right arm and elbow. Andy Murray, still clinging to the No. 1 ranking that he deserves only by virtue of mathematics, has played poorly all year and was prevented from defending his Wimbledon title partly because of a tricky hip injury.

This is a time of year when No. 9 Kei Nishikori and No. 10 Milos Raonic tend to shine. But Nishikori, who often has been sidelined, had his summer plans interrupted by yet another injury (back) heading into Wimbledon and lasted just two rounds at the grass major. Raonic, injured off-and-on, has handed opponents four walkovers in the past 12 months.

This week in Washington, mercurial Nick Kyrgios lasted just 12 games before he retired with a bum hip. The injury has caused him to retire during three consecutive tournaments, including Wimbledon. Maria Sharapova may match his hat trick.

Eager to resume her career in April following a 15-month suspension for doping, Sharapova has played just 10 matches this season. The ongoing event at Stanford, where she was given a controversial wild card, was just her fourth tournament this year. But it also became the second consecutive one in which she was forced to quit with injury. This time she has a sore left arm. She has also pulled out of Toronto, another Premier 5 that begins next week.

"I feel like I'm playing catch-up against everyone who's had a head start, all that matters is that I keep playing," Sharapova told reporters after her untidy three-set win over No. 80 Jennifer Brady in her first match at Stanford.

Easier said than done.

A year ago, Wawrinka rode his huge groundstrokes all the way to the US Open title. It was Wawrinka's third major, and it solidified him as a fixture in the game's elite, along with the Big Four.

Wawrinka, though, has struggled since reaching this year's French Open final, where he was pummeled by Nadal in straight sets. The Swiss has played only two matches, including the opening round of Wimbledon, and lost both.

Now he and Djokovic will have all kinds of time to heal and refresh themselves mentally, as Federer did one year ago.

At the rate things are going, there will be nobody left to contest the fall events. Those Asian and European tournaments have fought a losing battle against the rising tide of banged-up or burned-out players for years. It's a detail easily lost in the celebration of all those 30-and-over performers who are either bulletproof, wiser or simply tougher than their peers.

But there's more going on, especially with Serena Williams sitting out the year to have a child and Federer and Nadal enjoying a post-Wimbledon hiatus.

The top seed hasn't won once in the 10 (mixed clay and hard-court) ATP and WTA events held since the end of Wimbledon. Some top players have unexpectedly struggled against insurgent journeymen or rivals they usually crush. Perhaps it was an omen when lucky loser Andrey Rublev, one of the highly touted #NextGenATP players, won the Croatia Open.

That stable of NextGen players has been increasingly active and visible lately. In Washington, where No. 6 seed and defending champion Gael Monfils has already been eliminated by a qualifier ranked No. 200, Daniil Medvedev upset No. 14 Steve Johnson, and fellow NextGenner Tommy Paul of the U.S. took care of No. 7 seed Lucas Pouille.

Meanwhile, at Los Cabos this week, 19-year old American Taylor Fritz got off to his best start since the winter, with wins over Mikhail Kukushkin and No. 3 seed Albert Ramos-Vinolas.

In Atlanta, four promising American acolytes were stacked up in conventional slots in the draw. No. 348 Christopher Eubanks defeated Fritz in the battle of wild cards, while No. 8 seed Jared Donaldson dispatched No. 83 Ernest Escobedo. Eubanks than eliminated Donaldson, only to fall to a player who once was viewed by some as the potential savior of the U.S. game, Ryan Harrison.

The European newbies are restless as well. In the ongoing event in Kitzbuhel, 21-year old Austrian wild-card Sebastian Ofner has backed up his upset of top-seeded Pablo Cuevas by making the semis. Add that to the wins compiled in recent weeks by Rublev, Karen Khachanov and even Kyle Edmund, who upset top-seeded Jack Sock in the third round in Atlanta.

Young and old alike seem eager to make hay while the sun shines, before those two thunderheads called Federer and Nadal bear down on them starting next week in Montreal.