Christina McHale, energy sapped and unable to train properly, knew something was wrong last spring. Exercises McHale once did easily were becoming harder and getting through matches was proving difficult, too.
It was a surprise, since the young American prospect was considered a player with good stamina and has already earned a reputation for wearing down her opponents, not vice versa.
After a first-round victory at the French Open, McHale felt particularly exhausted.
"I won 6-4 in the third set, but I remember saying to my coach that I feel like I've just been hit by a bus," the 21-year-old recalled.
At first McHale thought it was a sinus infection. Then a couple of months later, following the Olympic tennis event at Wimbledon, McHale picked up a stomach bug and had to go to hospital. Her ordeal dragged on.
"I kept going back to the doctors because I wasn't getting better," McHale said. "I was still feeling very low on energy and they were like, 'No, you should already be over the stomach virus.' So then they started doing more tests, and that's when they found out what it was."
The diagnosis was mononucleosis, a viral illness that can linger for weeks, months or even years. It has earned the nickname of the "kissing disease" because it can get passed from one person to another through saliva. Fortunately for McHale, her bout was coming to an end.
McHale, though, isn't the only tennis player in recent years to be afflicted with mono or the name it's also known by, glandular fever.
Others on the list
Roger Federer, Andy Roddick, Robin Soderling, Mario Ancic and John Isner -- all top 10 players at one stage or another -- have been struck down, while Jarmila Gajdosova, Heather Watson, like McHale a player with promise, revealed she had mono in April.
Soderling and Ancic weren't as lucky as the likes of Federer, McHale and Watson, who hopes to return to the tour by this month's French Open, which starts this weekend. Indeed the severity of cases varies, as does an individual's capacity to fight off and cope with infections.
Soderling, the French Open finalist in 2009 and 2010, hasn't played since 2011 and it is looking increasingly likely that he won't ever come back.
The Swede with the massive forehand -- who handed Rafael Nadal his only defeat at Roland Garros -- started to feel unwell in the spring of 2011 and later said it was a mistake to compete at Wimbledon that year.
Ancic, hailed as a potential winner at Wimbledon after reaching the semifinals in 2004, attempted to play through his flu-like symptoms during a Davis Cup series against Germany in 2007.
He said he felt so dizzy in his singles opener he missed a ball completely, but he still contested the doubles a day later.
He was ready to play the deciding fifth rubber if needed, although with Germany already clinching the tie he was replaced by a young Marin Cilic.
"God saved me," Ancic, known for his work ethic and willingness to play through injuries, said in an interview in 2007.
Ancic re-emerged on the tour but was never the same and a teary-eyed Croatian had to retire two years ago aged 26.
Andy Murray, the current world No. 2, feared he had mono four years ago, and it's an illness he's still wary of, telling the Daily Telegraph in March: "You can get run down and end up missing two or three months of the year because of an illness.
"Your immune system gets run down and then you lose weight. It's happened with a lot of guys with glandular fever the last few years so it's something everyone has to look into."
Continuous travel, training
Former pro Justin Gimelstob isn't surprised that tennis players are susceptible.
The players have to, at times, switch continents on a weekly basis, and they travel 10-11 months in a year.
Unlike golf, cricket or Formula One, others sports that require continual global travel, the players also push themselves to the limit physically.
The career of Gimelstob, who now commentates and serves as a player representative on the ATP World Tour's board of directors, was blighted by a back injury.