Do the legwork
Ultimately, tennis players need fast, powerful legs.
"Players are hitting the ball harder - that means that you're going to have to get to a faster ball," says fitness trainer Gil Reyes, who has worked with the players like Sania Mirza and Sorana Cirstea. "That puts everything on the legs, and I mean everything. You also have to work on your lower back, and on your shoulders and wrists, but they are all secondary to the legs."
Strong is beautiful
Strength training not only adds power to a player's shots but it also enhances her control.
"You want to be able to hit the ball just as hard in the final set as you did in the first set," says Martina Navratilova. "When you lose power, you lose control, and you'll start missing more."
Suffering for success
Perhaps players could once rely on talent alone but that is plainly no longer the case. Players are aware of the suffering they have to endure when working on their fitness.
"This sport is just 10% talent - the rest is hard work," says Heather Watson. "When working on my fitness, I've never felt sick but I've sometimes been working so hard, both mentally and physically, that I've felt as though I'm about to cry my eyes out."
For all the anguish that players go through when working out, they know that physical preparations can also help to give them a mental edge on the match court.
"Going into every match, I believe I'm in better shape than my opponent," says Sloane Stephens. "That gives me an advantage before the match even begins as I have the confidence to play my best."
All year round
While a good off-season training block is crucial, players also top up their fitness during the year.
"Halfway through the year, I take a break from competition and work hard on my fitness and then I'm fit for the rest of the year," says Caroline Wozniacki.
Know your game
Players adjust their fitness regimes depending on their natural games - if they like to rush into the net, rather than sticking to the baseline, they are going to need a different type of physical conditioning.
"If you are going into the net a lot, you have to be in really great shape," says Navratilova. "That takes a lot more out of you than just staying on the baseline and running from side to side. You need a lot more energy going forwards and backwards than moving from side to side."