Jack Sock and John Isner gave the United States a 2-0 lead over Croatia on Friday in the best-of-five Davis Cup quarter-final.
Sock overcame a two-set deficit to beat Marin Cilic 4-6 3-6 6-3 6-4 6-4 and the 6-foot-10 Isner topped Borna Coric 6-4 6-4 6-3 in the second match on the Tualatin Hills hard court.
The US can wrap up the Davis Cup tie Saturday with a win in the doubles match between twins Bob and Mike Bryan and Ivan Dodig and Marin Draganja.
Two reverse singles matches are scheduled for Sunday.
The US-Croatia winner will advance to the semi-finals, playing the Czech Republic-France winner in September.
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The US is 157-3 when leading 2-0 in Davis Cup competition.
And the last time they blew a 2-0 advantage was the 1994 semi-finals against Sweden.
Sock, No. 26 in the world, gave the US the lead, though he got off to a sluggish start, as Cilic won the first two sets.
But the 23-year-old Sock, playing in his first career home Davis Cup tie, bounced back to tie the match at two sets each, then claimed the dramatic fifth set by breaking Cilic in the ninth game.
Sock finished the three-hour, 13-minute match when Cilic, No. 12 in the world, hit his final shot into the net.
Isner, dominating with his big serve that reached as high as 137mph, had 23 aces to Coric's three in his three-set win.
The 31-year-old Isner, No. 16 in the world and playing in his 20th career Davis Cup match, broke Coric once in each set.
The US last hosted a Davis Cup match in 2014, when it lost in the first round to Britain in San Diego.
This is the fourth time Portland has hosted a Davis Cup tie.
The US is trying to break a nine-year Davis Cup title drought when it beat Russia 4-1 in Portland. Croatia claimed its only Davis Cup title in 2005.
The US has won a Davis Cup-high 32 titles, but only one since 1995.
