For Petra Kvitova, everything changed thanks to a single, simple text message. It was February, and the Czech tennis standout had just landed in Doha, Qatar, for the next stop on the hectic tennis circuit. Seven months after winning her second Wimbledon title and four months after clinching another Fed Cup, she ought to have been on a high. But something was wrong. She felt empty, listless and, most worrying of all, unable to explain why.

Kvitova was preparing for her second-round match against Serbia's Jelena Jankovic when she checked her text messages. Among them was one from her longtime coach, David Kotyza.

"I think it's a good idea to take a break," he said. "Otherwise, I don't know how long you are going to keep on feeling this way."

Petra Kvitova said she was burned out, mentally, after playing so much tennis. AP Photo/Tim Ireland

Taken aback by the message, the 25-year-old knew something had to be done. She thought it over "for four days and for four nights" until she realized Kotyza was right. The pair had discussed her feelings in January during a tournament in Sydney in a conversation that had Kvitova on the verge of tears. Mentally and physically, she was exhausted.

"I felt terrible and just didn't understand what was wrong with me," she told ESPN.com. "I beat Jarmila [Gajdosova] and felt like I wanted to cry. David and I were in the elevator, and he said he wanted to talk to me. I said, 'Good, because I need to talk to you too,' and we were both thinking exactly the same thing. He said he saw me with the same energy I had when I was at the end of the season in Singapore. I said I feel like it's the end of the season already, and I just started. It was very worrying."

Kvitova missed tournaments at Indian Wells and Miami and spent much of that time asleep or lying on the beach. When she returned to tennis in April, her positive outlook had returned. She won the Madrid Open the following month, and despite early losses at the French Open and Wimbledon, she is enjoying tennis again.

"If I didn't take a break, maybe now I would be in bed and not wanting to wake up and go to work," she said in May in Rome. "Luckily, it didn't happen."

Kvitova revealed just last week that she is suffering from mononucleosis, an energy-sapping illness that does not respond to antibiotics. She said doctors told her she probably contracted it in the spring, and she is relieved to finally know why she was feeling so much fatigue, including mental fatigue.

Kvitova is not alone, and for many, it's far, far worse. High-profile athletes, from football to cricket, soccer to skiing, have publicly conveyed experiences with depression, anxiety, stress and panic attacks.

Recently, more tennis players have seemed willing to speak out while still on tour.

According to the Mental Health Foundation, about 25 percent of people worldwide will suffer from some form of mental health issue during the course of a year. There is no reason tennis, a sport that pushes athletes to the limits of physical and emotional stress, should be immune.

But could tennis be more vulnerable than other sports to problems of mental health? The enormous prize money at the top of the game means the very best are hugely rewarded. For those outside the top 100 -- where every week is a continued struggle to make a decent living -- winning and losing takes on increased significance. Throw in the unrelenting travel and the simple fact that tennis is an individual sport, and it gets even more stressful.

American Mardy Fish suffered cardiac arrhythmia in 2012, a setback that eventually led to a severe anxiety disorder that left him unable to leave his house at times. His return to the tour at Indian Wells in March 2013 was greeted with delight by his peers. Fish, who has played only sparingly since, remains on medication and undergoes therapy. He will hang up his rackets for good after his run at the Western & Southern Open, where he won his first-round match Monday, and then the US Open, his final tournament.

Mardy Fish's struggle with anxiety has kept him off the tour for a good part of the past three seasons. Kevin C. Cox/Getty Images

"I had tennis taken away from me because of it," Fish said to ESPN.com by phone on the eve of the French Open. "I had my job, which I loved to do, which I was really good at. I was at the top of my career, and I had it all taken away because of a mental illness."

Fish, who broke into the top 10 in 2011, said he is not bitter. He speaks of his illness in a bid to help others -- and also himself.

"Speaking about it was very therapeutic for me," Fish said. "That was one of the things I really struggled with in the beginning -- not talking about it, sort of holding it in. The more and more I spoke about it, the more I found out how many people deal with it, the more I read about it and researched it, the more you start to realize how many Americans deal with some sort of mental illness on a daily basis. That gave me comfort."

Fish was compelled to speak because he hadn't had an example of a high-profile athlete suffering from mental illness with whom he could confer.

"I wanted to be that success story, [someone] who people could lean on, knowing that even peers and colleagues of mine who hadn't come out with it or didn't know much about it have probably had it and we don't know," he said. "I'm obviously struggling to go out on my own terms. I want very badly to have a success story at the end of it, at the end of my career and say, regardless of how many matches I can win this summer, I want to go to the US Open -- for that be my final event -- and say I went out on my own terms, instead of it being taken away from me in Winston-Salem in 2013."

Many players, past and present, have suffered, often in silence, from feelings of anxiety, stress and depression. It's a familiar story, albeit one largely kept behind closed doors in tennis, lest anyone sniff weakness in a brutally competitive sport.

But talking about it in public helps others, including the sport's governing bodies, realize mental health is a growing concern in an ever-stressful world.

Dr. Nerina Ramlakhan, who works at the Nightingale Clinic in London, works with athletes who present a number of mental health symptoms, especially ones that affect sleep. Tennis players, she says, are as susceptible to mental health issues as athletes in any other sport, if not more so.

"If anything, [players] have to be aware of it because by virtue of their personalities -- typically, the sport attracts people who are used to being in control, extremely professional -- they want to be at the top of their game, so they put a lot of pressure on themselves," Ramlakhan said. "When I've worked with athletes who start running into problems, perfectionism is something that can get in the way of their health. To do well at the game, they have to be perfectionists to a degree, but you have to know where to draw the line, when to say, 'That's good enough. I just have to leave that for now,' rather than continually beating yourself up."

Cliff Richey, a former U.S. Davis Cup player who reached the US Open and French Open semifinals and Wimbledon quarters, is now most known for his battles with clinical depression, which had become so bad he required medication. Richey said he has reached out to many players, past and present, to try to help them.

Chris Evert says she suffered panic attacks after she stopped playing on the tour. ALLSPORT

"I've never tied a match yet. You either win or you lose. You're high or low," said Richey, who wrote "Acing Depression, A Tennis Champion's Toughest Match," a book depicting his often tumultuous life. "There's no getting around that if you really care about what you're doing. You have constant travel and jet lag, sometimes poor sleep. Sometimes there is some alcohol and drug abuse. People don't want to admit it, but we're all humans, and we know what goes on.

"[There's] the stigma that exists, particularly inside the sports world, because you need to be self-sufficient and mentally strong, and you don't ever want to cop to maybe having a problem in that way, and you've got, a lot of time, family dysfunction, and you have isolation."

Some players, such as Andre Agassi, Monica Seles and Pat Cash, have touched on the subject of mental health and depression once their careers finished. Victoria Azarenka and Heather Watson, meanwhile, have spoken of struggles they had when injuries temporarily forced them off the tour.

Canada's Rebecca Marino, who reached No. 38 in her career, quit the tour in 2013 because she was suffering from depression. Former women's world No. 1 and current ESPN TV analyst Chris Evert says she had panic attacks after she finished her playing career.

"I went from a tennis career right to being a mom, and that was a lot of highs," she said. "I think that I really didn't make that adjustment from tennis to life. I went from tennis to being a wife and a mom, and maybe that is life, but I didn't have a lot of time to come back to earth. I didn't start to feel any kind of anxiety about it or depression until all of a sudden my kids were grown up. Two out of three of them were out of the house, and then I was alone, and I had to face myself. You just do a lot of work, you get a lot of therapy.

"Anxiety is related. It can make you physically ill. I had panic attacks. ... I kept everything in for so long, I was that way about my life and my tennis. I was that way on the court, but after I stopped playing, it had to come out. Everything that goes in has to come out somehow."

Both the ATP and WTA Tours have assisted players who are or have been struggling. Although the ATP Tour does not have a formal, in-house staff psychologist like the WTA does, it can refer players for evaluation and treatment.

"Our tournament physicians and physiotherapists are in continual contact with players throughout the year," ATP spokesman Simon Higson said. "Were a player to express psychological concerns, we have an infrastructure that would refer them to the appropriate consultant. In situations where our physios and tournament physicians are concerned about a player's mental, emotional and psychological health, we would recommend the player seek treatment and assist in the initiation of the appropriate care. Our physios and tournament physicians work very closely with the players, and if they see any warning signs, they would take action in getting the player the appropriate help."

The WTA's in-house specialist, Kathy Martin, helps players adjust to the unique nature of the tour. The senior director for athlete assistance believes tennis mirrors life.

"Life stress can happen to anybody, like a death in the family, someone struggling with cancer, people moving house -- all those things are what human beings go through," Martin said. "Just because you pick up a tennis racket doesn't mean you've got a magic shield that protects you from all of that."

Martin meets every player who joins the WTA Tour, expanding on a support system she first began creating in 1995. Players are educated about mental health, especially at the beginning and end of their careers, when adjusting to normal life can be tough.

"There are a lot of preventative programs that we have in place to assist the players, particularly when they're coming in, when they're younger, when they are making a transition into the environment and also at the other end, when they're making the transition out," Martin said. "That's a time when there's a lot of change going on."

Richey believes the work the WTA is doing could set an example for the rest of sports.

"That's one of the things maybe the ATP needs to do, if they don't already have it on board," he said. "Tennis could be such a beacon and give itself such credibility and really even promote the sport if they were leading in this area."

Fish, who said he knows of players on both the men's and women's side who are dealing with similar problems, said he will help anyone who wants it.

"It gets much easier to speak about it," Fish said. "And the more people want to talk about it, the more I say, 'Here's my cell phone number. Call me any time. Let's talk about it. Let's talk about your pressures and your fears.'

"It's nice for me that I can help friends, help colleagues and help them understand what I went through -- that it is normal, it can be beaten and that it is a daily struggle, but that you do find your niche and your bubble that you feel comfortable in, and it does get better."