Los Angeles - Jack Sock rallied to stun Marin Cilic 4-6, 3-6, 6-3, 6-4, 6-4 on Friday and put the United States on the road to a 2-0 lead over Croatia in their Davis Cup World Group quarter-final tie.
The triumph for the 26th-ranked Sock over world number 12 Cilic, the 2014 US Open champion, was followed by American John Isner's 6-4, 6-4, 6-3 victory over Borna Coric.
The formidable doubles duo of Bob and Mike Bryan can clinch the best-of-five match tie on the outdoor hardcourt at Tualtin Hills Tennis Club in Portland, Oregon, on Saturday.
Croatians Ivan Dodig and Marin Draganja are slated to take on the brothers, trying to push things to the reverse singles on Sunday.
Sock fired 17 aces against Cilic, who came into the tie trying to put behind him a five-set quarter-final loss to Roger Federer at Wimbledon.
But having forced a decisive fifth set, the American surrendered an early break that appeared to set Cilic on his way.
Sock, however, broke back in the fifth game, and while he couldn't convert three break points in the seventh, he gained the last break he needed in the ninth, when Cilic sent a high backhand volley wide.
"A little bit disappointing with that, just to lose that opportunity of getting that lead in the fifth set," Cilic said. "When I gave him a chance to come back I felt that he started to play also really well."
Sock got a little shaky serving it out, double-faulting twice before surviving two break points to seal the match after three hours and 13 minutes.
"I haven't played a ton of five-setters in my career," said Sock, who was playing his first Davis Cup match on home soil.
"Haven't come back from two sets to love down. To do that today, in front of the home crowd for the team, it feels extra special today."
Isner, ranked 16th in the world, fired 23 aces and fended off all eight break points he faced in a match lasting just under two hours.
Having withstood 54th-ranked Coric's early assault on his serve, which included break points in six of Isner's first seven service games, Isner broke the Croatian in the final game of each of the first two sets to take a comfortable lead.
After breaking to lead 3-1 in the third, Isner stayed the course, finishing off the match with a final ace.
"We're up 2-0, we're in a good spot," Isner said.
While the United States are the most successful Davis Cup nation ever, with 32 titles, they were ousted in the first round the past two years by Britain.
They last played in the semi-finals in 2012, when they fell 3-1 at Spain.
The winners of the best-of-five match tie will take on either the Czech Republic or France in the semi-finals. The Czechs and French were tied 1-1 after the opening day of their tie in Prague.
Davis Cup quarter-finals results this weekend:
At Belgrade (clay, outdoors)
Serbia 0 Great Britain 1
Kyle Edmund (GBR) bt Janko Tipsarevic (SRB) 6-3, 6-4, 6-0
Dusan Lajovic (SRB) v James Ward (GBR)
Nenad Zimonjic/Janko Tipsarevic (SRB) v Jamie Murray/Dominic Inglot (GBR)
Janko Tipsarevic (SRB) v James Ward (GBR)
Dusan Lajovic (SRB) v Kyle Edmund (GBR)
At Pesaro (clay, outdoors)
Italy 0 Argentina 1
Federico Delbonis (ARG) bt Andreas Seppi (ITA) 7-6 (7/4), 3-6, 6-3, 7-6 (7/3)
Fabio Fognini (ITA) v Juan Monaco (ARG)
Marco Cecchinato/Paolo Lorenzi (ITA) v Juan Martin Del Potro/Guido Pella (ARG)
Fabio Fognini (ITA) v Federico Delbonis (ARG)
Andreas Seppi (ITA) v Juan Monaco (ARG)
At Trinec (hard, indoors)
Czech Republic 1 France 1
Lukas Rosol (CZE) bt Jo-Wilfried Tsonga (FRA) 6-4, 3-6, 4-6, 7-6 (10/8), 6-4
Lucas Pouille (FRA) bt Jiri Vesely (CZE) 7-6 (7/2), 6-4, 7-5
Radek Stepanek/Adam Pavlasek (CZE) v Nicolas Mahut/Pierre-Hugues Herbert (FRA)
Jiri Vesely (CZE) v Jo-Wilfried Tsonga (FRA)
Lukas Rosol (CZE) v Lucas Pouille (FRA)
At Portland (hard, outdoors)
United States 2 Croatia 0
Jack Sock (USA) bt Marin Cilic (CRO) 4-6, 3-6, 6-3, 6-4, 6-4
John Isner (USA) bt Borna Coric (CRO) 6-4, 6-4, 6-3
Bob Bryan/Mike Bryan (USA) v Ivan Dodig/Marin Draganja (CRO)
John Isner (USA) v Marin Cilic (CRO)
Jack Sock (USA) v Borna Coric (CRO)