LONDON -- Heading into Wimbledon, the world's best tennis players, including defending champion Novak Djokovic, were taking extra precautions.
"I plan advance space, either by bringing half of a bag or an entire new bag," Djokovic said before his first-round match.
As a new dad to 1-year-old Stefan, was Djokovic saving room for extra diapers and toys? Hardly.
"It's for the towels that I take from Wimbledon," Djokovic said, smiling.
This is a man who has made $100 million in prize money -- and at least as much off the court, too.
"I try to sneak in an extra towel here and there during the match," he explained. "I'm sure the All England Club will forgive me."
British Sterling took a pounding in the wake of Brexit, but towels are the true currency at the All England Club. This on-court necessity has grown so much in popularity that the signature linens have become the No. 1 seller at the Wimbledon gift shop. Last year, they were the runaway top choice -- 19,500 for men and 9,500 for the women -- followed by the mini tennis ball key ring (11,830).
The towels given to men for their matches are considered the classic look: Wimbledon's signature colors, purple and green, with tennis balls and letters written in a color referred to as "buttermilk." The women's towel is redesigned each year; the 2016 version is pink and jade, with letters printed in neon yellow.
"The players really prize the Wimbledon towels," said George Spring, the court attendant manager at Wimbledon. "They're like souvenirs."
Part of Spring's role at the All England Club entails managing the 6,000 player towels that come through the tournament each year. On average, during the first week of the Championships, about 200 towels go out per day.
"We probably only get about 15 percent back," added Spring. "It baffles me."
Players are open about their intentions. When their time at the tournament ends, they take a small piece of Wimbledon home with them.
"The towels I use on the court and play with are more significant than one I would buy in the gift shop, because I played with it at Wimbledon," explained 2014 finalist Eugenie Bouchard. "They are unique. Everyone seems to want one."
Spring and his staff are well aware of the devious nature in which competitors pile up these highly sought mementos.
"When the players come on court, before the first shot is hit, they will put the towels in their bag, then ask the chair umpire for more towels," Spring said. "That is slightly cheeky because they're not even being used.
"They understand the system, which is frustrating."
Like most protocols at Wimbledon, the towel process maintained by Spring's staff is meticulous and consistent: Two Wimbledon towels wait for each player prior to their arrival on court. And if there's a rain delay? Two more fresh towels waiting for them -- guaranteed.
"One match, four towels," Bouchard said. "I just take as many as I can, because I know everyone wants one. Over the years, my collection has definitely gotten pretty big."
American Sam Querrey sat through multiple stoppages of play throughout his match with Djokovic in the third round.
"That's been the one bonus to all these rain delays," Querrey said. "You get two every time. I've got like 50 in my locker room for my friends and family."
That could be an Open era record.
When players have amassed enough towels for themselves, continuing to pursue them becomes a selfless act of kindness.
"I think I was able to make out with six of them in this last match," John Isner told reporters after his first-round win over Marcos Baghdatis.
"Shhhh," he added, smiling. "Then during my second-round match, there's two more right there. I'm dishing them out like candy right now."
For many players, like two-time Wimbledon semifinalist Jo-Wilfried Tsonga, the towel requests from friends and family have become so overwhelming, he has to distribute them on a first-come, first-serve basis.
"All my family and friends book the towels before the tournament," Tsonga said. "The first round is for me, the second round is for them. When I go out of the court, the fans ask for the towels. I have to say, 'Sorry, it's booked already.'"
American Madison Keys had similar issues.
"I am constantly getting text messages, 'Can you grab me a towel?'" said Keys, who reached the fourth round here. "For the most part, you can bring some, but I can't pack 75 towels and take them home. It's not going to happen."
As for Djokovic, his half-empty suitcase is typically jam-packed upon exiting.
"It makes a lot of people who are close to me back in my country happy," he said. "The towels are great memorabilia. I have plenty of towels from other Grand Slams at home as well, but I also give away a lot."
Accounting for the missing towels can be frustrating for Spring and his staff, but seeing the players walk off with the most prized possessions of the event, outside of the sterling championship trophies, also brings him a sense of pride.
"It's part of Wimbledon," he said. "It's a Wimbledon towel."