His coach, Rick Leach, a former No. 1 doubles player, suggested loosening up when playing with polyester strings.

WSJ: Roger Federer: With Tennis Rackets, He Strings Them Loose

Most pros now use at least some of the newer polyester-like monofilament string often called "polyester" or "poly" for at least some of their strings.

WSJ: Roger Federer: With Tennis Rackets, He Strings Them Loose