LOS ANGELES (AFP) - 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 No. 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 hard court 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.
Defending champions Britain shrugged off the absence of Wimbledon champion Andy Murray to lead their quarter-final against Serbia on Friday as heavy rain brought havoc to the tournament.
Kyle Edmund eased past Janko Tipsarevic 6-3, 6-4, 6-0 for first Davis Cup win at the Tasmajdan stadium in Belgrade, which is built on the site of a Roman-era quarry, before the scheduled second singles rubber was postponed until Saturday.
Serbia, the 2010 champions, are without world No. 1 Novak Djokovic, who pulled out of the quarter-final after his shock third-round exit at Wimbledon.
In Pesaro, torrential rain delayed the start of the quarter-final between Italy and Argentina by three hours. When they did get under way, Federico Delbonis defeated veteran Andreas Seppi 7-6 (7-4), 3-6, 6-3, 7-6 (7-3).
The scheduled second singles rubber of the day between Fabio Fognini and Juan Monaco was pushed back to Saturday.
In Trinec, meanwhile, the Czech Republic and France were tied 1-1 after the opening rubbers as both No. 2 players beat the team leaders.
World No. 78 Lukas Rosol stunned 10th-ranked Jo-Wilfried Tsonga 6-4, 3-6, 4-6, 7-6 (10-8), 6-4.
Lucas Pouille, ranked 21st, then swept 50th-ranked Jiri Vesely 7-6 (7-2), 6-4, 7-5 to level the tie, which started with a minute's silence for the victims of Thursday's terror attack in Nice that left at least 84 people dead.
For the 32-time champions, American 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 could not 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.
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.
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," he 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.