@time: In just three weeks at the U.S. Open, this team is expected to string and restring around 8,000 tennis rackets, making sure the tension of each one is exactly what the tennis pros need. Sometimes, that means being on their feet and working late into the night. TIME talked to a veteran racket stringer from Ukraine who has worked at the U.S. Open 21 times, stringing rackets for legends like #RogerFederer and #SerenaWilliams. #USOpen #tennis