When players such as world No. 2 Simona Halep need to get some practice in, they’ve got their individual preferences. Some look for a friend or countryman. Some look for a player they will or will not face later in the tournament. Others seek players with a certain style. (Katherine Frey/The Washington Post)

When second-seeded Kei Nishikori and third-seeded Milos Raonic walked onto stadium court at the Citi Open to practice with each other Monday morning, they knew they could end up facing each other later in the week, and they chose to do so in sake of good preparation.

Before that, the second-ranked woman in the world, Simona Halep, chose to hit with fellow Romanian Patricia Maria Tig, the 134th-ranked player, saying she was willing to hit with anyone who was available. Jack Sock, the highest-seeded American, picked his practice partner, Nick Kyrgios, because of their close friendship.

There is no hard-and-fast rule for choosing practice partners; it varies by individual. Such is the unique nature of tennis, where players even warm up with their opponents before matches. Choosing a practice partner can be strategic — or not. Some players value hitting with their friends as much as they do preparing to play a left-handed hitter.

“I usually just practice with my closer friends. That’s usually how it goes,” said Sock, who takes the more relaxed approach when it comes to picking partners. “[My] coach will ask me maybe with who I want to practice with, but I’ll just practice with one of my buddies like John [Isner] or Nick or somebody like that.”

The first step to securing practice partners is paying a visit to Alex Cordier, down the stairs and in the bowels of stadium court. Here, Cordier has been the practice courts coordinator at the Citi Open for 15 years. His days consist of filling out paper spreadsheets as he pieces together a practice time for each player at the tournament. And it’s not an easy task.

Kei Nishikori practiced against Milos Roanic fully aware that they might face off later in the tournament. (Tasos Katopodis/Getty Images)

“That’s why it’s in pencil,” Cordier said. “It is crazy because it changes a lot.”

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A handful of big-name players at the Citi Open have been practicing against equally ranked opponents, causing daily crowds to gather around the courts, waiting for the next big name to stroll by. On Monday, there were plenty of big practice court pairings, including top-seeded Dominic Thiem and fourth-seeded Grigor Dimitrov on Grandstand 1.

Unlike Sock, defending Citi Open champion Gael Monfils said his coach handles all of his practice schedules; he tries to figure out who is playing well, when he can practice with them and which hours work best. Three-time Citi Open champion Juan Martin del Potro said he has his own sparring partner for practice, but he enjoys practicing against the other players in the beginning of the tournament. Hitting partners aren’t uncommon on the tour as some players bring them to tournaments as part of their team.

Whether players have chosen partners or not, they can’t randomly choose a practice court and play whenever they want. It’s Cordier’s job to manage the times and locations, and scheduling the courts during prime times can get tricky. The rule is that top-seeded players don’t get any priority or special advantage when it comes to choosing practice courts and times. Only players who have a match that particular day receive preference, and sometimes that doesn’t go over well.

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“If we are fully booked, [players] will start nagging and say it’s not fair, so I’ve realized over the years you have to forget the names and be fair,” Cordier said. “At the end of the day, if you are fair, they all understand — though they might be [ticked] off in the beginning.”

As for choosing specific players to practice against, Cordier said players routinely discuss it among themselves, as Sock does with Isner and Kyrgios, but often if they don’t have a specific player in mind, they will write down that they are “looking.”

Sometimes players will write down that they are looking for a right-hander, a left-hander or someone on the opposite side of the draw. But Cordier said player requests aren’t that common — they just need someone who can hit. It’s usually the coaches who are the most “fussy,” and the players will usually go along with whatever works best for their schedule.

Over the years, Cordier has gotten to know the players and found out their quirks as he tries to match players with those who are “looking” throughout the tournament.

“You’ve got to know the players and not only by name but how they play, like, what they enjoy, what partner they enjoy playing with — and those they don’t — and other players they have problems with,” Cordier said.

Oftentimes, if professional players aren’t available, younger players or crew members at the Citi Open jump in to take their place.

“The year [Alexandr] Dolgopolov won the tournament [2012], that’s exactly what happened,” Cordier said. “It was the first practice, there was no pro here, and so he needed someone to hit with, and we were like, ‘Yeah, we have a guy from the crew. He can hit well.’ And it was such a good practice for him that he requested him for the entire week, and he ended up winning, and the same thing happened last year with Monfils.

“It happens a lot where either two pros stick together throughout the week or the opposite happens, like, ‘Never give me that guy again; he can’t hit a ball.’ ”

Romanian Monica Niculescu has an unusual style of play that has caused practice partner problems in the past. Niculescu has used a forehand slice, not a topspin forehand, for most of her career. The forehand slice is a constant in her play, and when it comes to practice, not all players need or want to face it in preparation for their upcoming match.

“If someone wants to play with me, they do,” Niculescu said. “If not, they leave me looking because I play different and maybe the girls are playing against girls who play flat and strong, and I play mixed, so I always put looking, and if someone wants to play with me they will play. If not, usually I play with my coach.

“It’s not that easy, but it’s fun.”