For globetrotting tour insiders, Chip, Pierre and Bruno manage to keep a low profile, not to mention a lot of feet on the ground.

They jet off to tournaments. They carry special passports. They hang out in the players’ lounge, show up at press conferences and pass on a first-name basis. But you won’t find them on any official credential, guest or entry list.

Chip, Pierre and Bruno walk on all fours. They are dogs.

Like coaches, nutritionists and conditioning experts, canines are increasingly part of the ever-expanding modern tennis entourage; a lavish—and some would argue, outlandish—ingredient in the smorgasbord of circuit life.

No shortage of top players travel with their tail-waggers, from Serena Williams, Novak Djokovic and Caroline Wozniacki (regularly) to Bethanie Mattek-Sands, Andy Murray and Svetlana Kuznetsova (occasionally).

To owners, the headaches of criss-crossing continents with a dog in tow are outweighed by the gift of unconditional affirmation despite life lived out of a suitcase. They are a touchstone of domestic stability and companionship on tennis’ long and lonely road.

“It’s so comforting,” said Williams, who often refers to her 10-pound Yorkshire Terrier, Chip, as “my son.”

“We don’t see them as just an animal,” echoes Djokovic, whose toy poodle, Pierre (being carried, at right), is closest to his wife, Jelena.

As Wozniacki says of her Pomeranian puffball, Bruno, who never leaves home without his European Union doggy passport: “It’s totally worth it. He’s always happy to see you.”

Weathering far-flung time zones with a mutt or, more typically, a pint-sized purebred, is not as expensive as it might sound. The added fees for flights, hotels and food amount to a few thousand dollars a year, said several players. That’s pocket change to multi-millionaires like Williams and Djokovic.

But the potential hassles are many, and are occasionally emotionally costly. Animals end up stranded at customs by paperwork snafus. They relieve themselves in busy airports or hotel lobbies. They must be fed, walked and exercised, all while negotiating the rigorous demands of elite tennis.

“I love dogs,” says Djokovic, whose transition to fatherhood last year did not deter him from adding a second toy poodle, Tesla, to his growing brood. “But logistically it’s not easy, especially when you travel on the opposite side of the world.”

Indeed, dog-care duties often fall on other members of a player’s team. Significant others, parents and hitting partners all pitch in. But players also go to great lengths to make their pets content, and it is not unusual to see them out strolling with their dogs on streets and in cafés across the globe. Djokovic recently found himself sneaking 7-year-old Pierre into the cramped lavatory of a transatlantic flight so that the little poodle could do his business. He positioned Pierre over the seat. He sweet-talked. He stroked his fur. He flushed. Pierre did not oblige.

If the idea of Serbia’s best-known citizen at 33,000 feet bent over the toilet coaxing his dog to go sounds absurd, Djokovic does not disagree. “What can I do?” he says, sounding chagrined as he explains commercial airlines’ in-bag policy, even for flights that can last 14 hours. “It’s the only place I can let him out.”

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Dog-friendly travel is generally confined to Europe and the U.S., where entry and quarantine laws are less strict compared to Asia and Australia. The one exception is England, where entry remains a challenge, players say.

A few years ago, Kuznetsova tried to bring Dolce, her 50-pound American Bulldog, to Wimbledon. The two-time Grand Slam champion hit a snag on arrival and the notoriously strict U.K. authorities placed Dolce in quarantine.

Kuznetsova was beside herself. She left the tournament to visit him every day at a small, dirty facility next to Heathrow Airport. The fees ended up costing her more than her hotel in London. “It was a mess,” she says. “I was crying so much.”

By definition, the best-traveled dogs are toy breeds. They rarely exceed 20 pounds, so they can ride inside airplane cabins. But there are exceptions.

American Bethanie Mattek-Sands has taken to the road with Ruger (at right), a 140-pound South African Mastiff-like breed called a Boerboel. Named for the Ruger Magnum firearm, he entered her life when she and her husband, Justin, started dating. Justin had acquired Ruger just before they met.

Ruger, understandably, has never flown. But he has accompanied the couple on road trips from their Arizona base to tournaments in California. One summer, Ruger visited several tour stops on the East Coast and in Canada.

Despite his intimidating size, Mattek-Sands says Ruger is a big baby. He doesn’t thrive away from the familiar surroundings of home. He won’t eat at first. He’s startled by any small noise. He must sleep with his toys. “He kinda freaks out a little bit,” she says.

Mattek-Sands says she and her husband often feel like “absentee parents” because they leave him in the care of others so often. And though he’s high-maintenance, Ruger is a sweet, obedient, playful pooch that sleeps with them in bed. “He’s like another person to cuddle with,” Mattek-Sands says. When he leaves a mark, however, it doesn’t go unnoticed. “They’re huge,” says Mattek-Sands of Ruger’s droppings. “You won’t miss them.”

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Not everyone views the decision to tote animals around the world as a worthwhile inconvenience. Dominika Cibulkova’s parents were none too pleased when the Slovakian started to travel regularly with her two Yorkshire Terriers, Woody and Spajky.

Kuznetsova says her father, a renowned coach of several Olympic cycling medalists, went “nuts” when she began to bring Dolce to tournaments. “He said, ‘You cannot do this! You’re an athlete! You need to focus! You will disturb your sleep,’ and blah blah blah.”

While modern travel and lucrative prize money have made such extravagances possible, the idea of folding a pet into the most peripatetic of sports strikes others as doggone crazy.

“It’s not for me,” says animal lover Agnieszka Radwanska, with a dismissive roll of the eyes.

Jelena Jankovic, who has a Maltese named Stella at her home in San Diego, goes a bit further. She says it’s unfair. “Being in a bag all day is not fun,” the Serbian says. “I prefer my dog to be at home and live a normal life.”

Still, others view it as a luxury for the elite. “I think that only would work if you’re one of the top players flying privately,” says dog-loving American Tim Smyczek. “That would be a goal of mine—to afford a dog on tour.”

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Most dogs coexist amiably on tour, but tensions can arise. Mattek-Sands says Ruger became embroiled in a “standoff” with Kuznetsova’s Dolce when they crossed paths in a hotel lobby.

“They had this little alpha male thing going,” says Mattek-Sands. “I’m just glad Justin was holding him because he’s strong. If he really wants to take off, I have no chance.”

Some dogs keep to themselves. Maria Sharapova’s Pomeranian, also named Dolce, once appeared in a series of Canon ads with the L.A.-based superstar but is not one to mingle. “He’s unsocial,” Sharapova says, “kind of like his owner.”

Social or not, players face daily inconveniences in exchange for their loyalty. Dogs disrupt sleep by barking at strange noises. They whine. They snore. They get sick. They act out. Kuznetsova’s dog once chewed up her favorite sunglasses. But that’s not his main offense. “He farts a lot,” she says.

Food is another issue. Simple kibble does not always make the cut. Some players cart special concoctions with them. Diet-crazed Djokovic says he cooks rice, carrots and beef for Pierre and Tesla. Sharapova’s tiny and spirited dog is on a raw diet. “It’s a Russian thing,” she says.

And though no toy is more ubiquitous to dogs than the tennis ball, not all ATP and WTA canines are necessarily fans. Some chase them; some disdain them. “If I throw him a tennis ball he just looks at me and says, ‘OK you run and get it,’” Djokovic says of Pierre.

Wozniacki, on the other hand, trained Bruno not to chase tennis balls, or any yellow ball. “We said no because when I’m practicing I want him to be at the side of the court,” she says.

On the flip side, dogs jump for joy, wag tails, lick faces and gaze up adoringly. Players who don’t take them on the road describe giddy reunions when they reunite back at home. “My best moment is when I come from a trip and I open the door and my dog starts running in circles,” Jankovic gushes.

“I think we all need distractions, something to look forward to when we come back home—the unconditional love you get from pets,” says Nadia Petrova, whose French Bulldog, Lulu, stays at her base in South Florida. “They are the best therapists.”

And while there are challenges, the bond between master and dog runs deep, even for some of the world’s least stationary athletes. “We’re all one family,” says Djokovic.

Williams says she would travel more regularly with Chip (above) and her other dog, a Maltese named Lorelei, if she could. But even that would barely suffice, says the American great. “I could live with 100 of them if I had the time.”

Originally published in the July/August issue of TENNIS Magazine.