American teenager Jack Sock took a two-set lead over 22nd-seeded Florian Mayer at the U.S. Open before Mayer quit the first-round match Monday after feeling faint and dizzy.
The 19-year-old Sock, of Lincoln, Neb., won the U.S. Open boys championship in 2010, becoming the first American to take that title since Andy Roddick, 10 years earlier. Last year, Sock won his first-round match before running into Roddick, who beat him in straight sets.
Sock teamed with American Melanie Oudin to win the mixed doubles title at the U.S. Open last year.
He came into this year's tournament ranked 248th and without a win over a top-50 player.
Sock was ahead 6-3, 6-2, 3-2 when Mayer retired.