Roger Federer slumped to his first Davis Cup defeat in nine years, losing 4-6 6-3 7-6 (7-4) 6-2 to John Isner as the United States opened up a 2-0 World Group lead over Switzerland.
Isner secured victory against the world number three with three return winners as the 32-time Davis Cup champions moved closer to a quarter-final spot.
"I missed more opportunities than he did and that cost me," Federer said.
Earlier, Mardy Fish beat Stanislas Wawrinka 6-2 4-6 4-6 6-1 9-7.
Federer won the first set without dropping serve, before it was broken in the sixth game of the second set as Isner levelled the match.
Sixteen-time Grand Slam winner Federer could not convert break chances in the sixth and eighth games of the third set, taking just one of his 12 opportunities in the match. Isner made the Swiss pay by winning in the tie-break.
The fourth set turned on the fifth game when Isner battled back from 0-40 down with three big serves and two thunderous forehand winners. Federer never recovered and Isner won four consecutive games, ending the match by hitting a cross-court backhand winner.
"I thought he played great - he played it tough and served great when he had to," Federer added.
"It's been a difficult day for us [but] we still have a chance to go through. I'm going to enjoy the challenge."
The United States can eliminate Switzerland in Saturday's doubles when Mike Bryan and Ryan Harrison face Federer and Stanislas Wawrinka, the 2008 Beijing Olympic gold medalists.