Roger Federer, of Switzerland, reacts after winning a game against Frances Tiafoe, of the United States, during the U.S. Open tennis tournament, Tuesday, Aug. 29, 2017, in New York.

NEW YORK (AP) — Roger Federer already has dropped two more sets at the U.S. Open than he did during his entire two weeks en route to the title at Wimbledon.

Worried about a recent back problem that kept him from getting ready the way he usually does for a Grand Slam tournament, Federer had to overcome an early deficit and a late lapse Tuesday night to edge 19-year-old American Frances Tiafoe 4-6, 6-2, 6-1, 1-6, 6-4 in a compelling first-round contest under the Arthur Ashe Stadium roof.

He missed last year’s tournament, when the retractable cover made its debut, because he took off the second half of 2016 to let his surgically repaired left knee and a bad back fully heal.

In 2017, he is 36-3 with five titles, including the Australian Open and Wimbledon, where Federer won every set he played across seven matches while collecting his record eighth trophy at the All England Club and 19th Grand Slam championship overall.