Last Tuesday morning, on the second day of the U.S. Open, Nate Ferguson and Ron Yu were in Yu’s hotel room, on the East Side of Manhattan. Ferguson was seated on the edge of one of two double beds, while Yu was stringing the last of nine racquets that their client, Roger Federer, had requested for his first-round match, which would be played that night. During matches, Federer changes racquets each time the balls are changed, which happens after the first seven games and then every nine games thereafter. Nine racquets would be more than sufficient to see him through his opening match. The already strung rackets were arranged in a row against a dresser. Each had a small piece of white tape on its throat indicating the day it had been strung and the level of string tension. Federer had texted Yu the night before with his specifications: he had asked for three racquets to be strung at twenty-six kilos (“Roger wants it in kilos, not pounds,” Ferguson explained), five at 26.5, and one at twenty-seven. Spools of racquet string and rolls of grip tape were strewn across the dresser.

Yu, who was dressed in khaki shorts, a light-blue T-shirt, and sandals, was working at one of the four Babolat Star 4 stringing machines that he and Ferguson had brought to New York (they have two other stringers working with them during the Open). Although the Star 4 dates back to the nineteen-eighties and is no longer in production, they continue to use it because it is light and easy to travel with. “Once these break down, we’re retiring,” Yu joked, as he ran a strand of natural-gut string through the top of the racquet head (Federer was using natural gut for the sixteen main strings and polyester for the nineteen cross strings). After Yu finished, he clipped ten tiny plastic string savers into the string bed of each racquet. Federer likes the string savers because they supposedly reduce friction. Yu, who estimates that he has strung racquets more than five thousand times for Federer, is skeptical. “I don’t think it does much,” he said. “But these guys don’t like to change things,” Ferguson added. After punching in the string savers, Yu stencilled a red Wilson logo on each racquet, wrapped them individually in long plastic bags, sealed the bags with blue tape, then stuck a large decal bearing Federer’s logo (a stylized “RF”) on each bag.

Yu, who is 47, and Ferguson, who is 51, work together under the company name P1—short for Priority 1—which is based in Tampa and does stringing and racquet customizing (gripping, handle modifications, weight adjustments) for some of the biggest names in professional tennis. In addition to Federer, P1’s clients include Novak Djokovic and Andy Murray. They make up three of tennis’s Big Four—Rafael Nadal, who missed this year’s U.S. Open because of a wrist injury, is the fourth—and often end up playing against one another.

Yu said that having clients on both sides of the net is good for business but too much of an emotional roller coaster for him. “I hate it,” he said. The P1 team travels to all four Grand Slams, as well as the nine Master Series tournaments. Players pay forty thousand dollars annually for stringing and customizing. “They pay us for peace of mind,” Ferguson said. P1 currently has around two dozen clients, all touring professionals. “They don’t want surprises out there,” he added.

Federer and the other players who use P1 have racquets restrung before every match. P1 used to regrip Murray’s racquets, but the British player now handles it himself. Ferguson and Yu don’t feel slighted. “Some guys like to do it because it gives them something to do before a match,” Ferguson said. All of P1’s clients are men. Ferguson said that, over the years, he has fielded inquiries from Serena Williams and other female players. “Once I told them the cost, I’d never hear back from them.” He said that the women generally use fewer racquets than the men do so they have less of a need for a full-time stringing service. Also, “Guys care more about their tools—their sneakers, the technical aspects of their racquets,” Ferguson said.

Ferguson, a tall, wiry Connecticut native, came to prominence, in the nineteen-nineties, as the stringer and racquet technician for Pete Sampras. Sampras was notoriously finicky—if the leather grip on a racquet handle shrank by a millimetre, he could feel the difference. “It was all about the compression for him,” Ferguson said.

“Pete was crazy about the racquet handles,” Yu said. “I’m glad I never had to work with him.”

Ferguson’s ability to keep Sampras satisfied—he had the gig for thirteen years, during which Sampras won fourteen major titles—earned him a reputation as a racquet-care genius. In 1998, he founded P1. Three years later, he hired Yu, who had been stringing for Sampras’ rival, Andre Agassi. Federer hired the duo on a trial basis at the Italian Open in May, 2004. Two months later, Federer successfully defended his Wimbledon title. A few hours after the match, Ferguson and Yu heard a knock on the door of their rented house. Federer, dressed in a tuxedo for the Champions Ball, had stopped by to let them know that he was hiring them full-time.

Federer’s longtime nemesis, Nadal, also frustrates Ferguson and Yu. Whenever he wins a big title, it usually comes at the expense of a P1 client. At tournaments, their motto is “A.B.R.”—anyone but Rafa. Nadal also frustrates them because he hasn’t become a client. Years ago, they approached his agent, Carlos Costa, offering their services. “It will never happen,” Costa told them. In tennis circles, it is widely agreed that no player has benefitted more from advances in racquet and string technology than the twenty-eight-year-old Nadal, and Ferguson and Yu figure that if anyone could use their help it would be him. “We could make his life easier,” Yu said. Still, Nadal has won fourteen Grand Slam titles, second only to Federer’s all-time mark of seventeen. “You can’t really argue that he’s doing things wrong,” Yu conceded.

By now, the first matches at the Open were under way. Ferguson was going to take a nap. He would be up late stringing racquets for Marco Baghdatis, a player from Cyprus. At some point, he would also have to pick up racquets from Ernests Gulbis. Gulbis is a mercurial player who has a habit of smashing racquets when matches don’t go his way. (“He’s a breaker,” Ferguson said. “He doesn’t just break one; he’ll break three or four.”)

Ferguson and Yu, who both quit playing tennis, years ago, almost never watch courtside. (“It’s two-plus hours; I’m not going to do that,” Ferguson said.) They said that the polyester strings, which can generate tremendous topspin, have made tennis less interesting. “It has changed; nobody comes to the net anymore,” Ferguson said. Yu nodded in agreement. “It has become a fitness sport, not a shot-making sport,” he said.

Yu, reclining on the other double bed, said that he would deliver the racquets to Federer at his hotel early that afternoon. He said that Federer usually offers him a piece of chocolate when he comes by (“he’s Swiss”). At Wimbledon last year, Yu dropped off some racquets for Federer and briefly found himself alone in the backyard with one of Federer’s three-year-old twin daughters, who was bouncing on a trampoline. “That was a lot of pressure,” Yu said. “I was thinking, ‘Please don’t hurt yourself.’ ”

Correction: This post initially misstated the number of Masters Series tournaments.