LONDON -- Novak Djokovic has been taken to five sets by Marin Cilic at Wimbledon this year. He dropped a set to Radek Stepanek. And he narrowly beat a seriously game Grigor Dimitrov in the semifinals. But his toughest challenger during these two weeks has not been someone across the net -- it's been the grass on his side of the court.
The Serb has been slipping, tripping and tumbling on the lawns of the All England Club, and he almost sent himself out of the tournament by rolling across the grass and banging his shoulder during his third-round match against Gilles Simon.
It was a poor imitation of the legendary lunging his coach Boris Becker used to do during his playing days. "We obviously need to work on my 'diving volleys,'" Djokovic quipped.
The problem, though, might not be Djokovic's technique as much as the shape of his rule-abiding footwear.
Typical grass-court shoes have pimpling on their soles to provide a better grip on this slippery surface, and most players use some variation of this design.
Grand Slam rules, however, do not allow this to extend around the edges of the shoe, saying "the foxing around the toes must be smooth." That works fine for players running on the soles of their feet, but that's not the only way many of them -- especially Djokovic -- cover the court these days.
The world No. 2 has pioneered the regular use of the hard-court slide, taking the clay-court move and somehow transferring it to gritty cement. The most flexible player on the men's tour simply tilts his ankles and skids across on the sides of his shoes, getting to balls that would otherwise be out of reach.
Grass, which provides less resistance, is even easier to slide on, but players require better traction to keep their balance and recover for the next shot.
A year ago, the thinkers at adidas came up with a design that also featured pimpling on the edges of the shoes, which would be expected to grip the court better while sliding. Attempting to circumvent the smooth-edges rule, they put a piping above the sole so the rough portions were still beneath the outmost edge of the shoe.
Djokovic, who wears adidas, wore the shoes through the Wimbledon quarterfinals before the issue was raised, with the Club telling him to bring the design into line before his next match. For the semifinals against Juan Martin del Potro in 2013, Djokovic had filed down the edges of the shoes and had noticeable problems with his movement, slipping repeatedly as he tried to slide into shots.
This year, his shoes conform fully with the rules, and the slipping problems have continued. After going down two sets to one against Cilic in the quarterfinals, Djokovic changed his shoes and won the next two sets comfortably.
"I wasn't really finding the balance in the third," he said. "I don't know if it was shoes or socks or whatever. It was very warm. I was sweating a lot, so I want to change it. I had just a better grip. I had better movement. Maybe it was just mental, but anyway, it worked."
With movement such a big part of his game, Djokovic is particular about the condition of his shoes and has changed shoes midmatch on other surfaces as well.
The 27-year-old will not say he is having problems adjusting to the new grass-court design but did note the crackdown during the first week.
"The rules are the same for everybody considering the footwear," Djokovic said. "Adidas shoes, that I've been using for most of my professional career, are quite good for the grass courts. They're the same like Andy Murray's or any other adidas player.
"In last couple of years, there were some, you know, talks and rumors about certain players having a little bit of an advantage with the sole of the shoe, but this year the referee's office made sure that all the shoes, regardless of the ranking of the player, is exactly the same and according to the rules."
It's not the first time a close look has been taken at Djokovic's grass-court shoes. In 2008, he swapped his adidas kicks for Nike, covering up identifying marks on the competitor's shoes.
But he has stuck to his sponsor's offerings since, including his victory at Wimbledon in 2011. Djokovic has switched clothing companies, going in 2009 to Sergio Tacchini and then in 2012 to Uniqlo, but continued lacing up with adidas because neither company produces shoes. In 2013, he signed a shoe deal with adidas that has been renewed this year.
The Grand Slam rule on grass-court shoes, however, has stayed the same even though the way players move has changed. The restrictions take into account the effect on the surface as well as the amount of grip the players need. But the apparent increase in the number of falls and injuries could prompt another look at the designs allowed.
Otherwise, whatever he wears, Djokovic will be walking a fine edge on grass.