PORTLAND, Ore. (AP) Jack Sock needed a little time to get up to speed Friday in his first Davis Cup match.
Once Sock got his serve straightened out, the U.S. raced to a 2-0 lead over Croatia in the best-of-five quarterfinal tie.
Sock overcame a two-set deficit to beat Marin Cilic 4-6, 3-6, 6-3, 6-4, 6-4, and John Isner topped Borna Coric 6-4, 6-4, 6-3 in the second match on the Tualatin Hills hard court.
The U.S. 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.
''Our mindset is we're happy,'' U.S. captain Jim Courier said. ''For us, it's about putting our hard hats on and let Bob and Mike do what they do. I like our chances.''
The U.S-Croatia winner will advance to the semifinals in September to face the Czech Republic-France winner. The United States is seeking its first semifinals appearance since 2012.
The United States is 157-3 when leading 2-0 in Davis Cup competition. The last time the U.S. blew a 2-0 advantage was the 1994 semifinals against Sweden.
The Bryan twins are 10-4 in Davis Cup doubles competition.
The 23-year-old Sock set the tone Friday, though it took a while to get going. Cilic, No. 12 in the world, rushed to a two-set lead after breaking Sock three times in the second set. But Sock also had his moments during the first two sets, and he used that, along with a bit of inspiration from Courier to fuel a comeback.
''I had a ton of chances in the second. I didn't get blown off the court. I just had to make a few smarter decisions on the first serve and returning,'' Sock said.
Courier noted that Cilic might be struggling a bit mentally after the 27-year-old Croatian blew a two-set lead to Roger Federer in the Wimbledon quarterfinals less than two weeks ago.
''I was using that as a little bit of a carrot to keep Jack in there,'' Courier said.
The big-serving Sock bounced back, using an early service break to win the third set. Sock, No. 26 in the world, then pulled even, winning the fourth set by claiming the final two games.
Cilic, the 2014 U.S. Open champion, said it wasn't momentum swinging in Sock's favor as much as the young American simply improved his play.
''Jack started to serve really well ... he was not giving me too many chances in the return game,'' Cilic said.
Sock won the fifth set after breaking Cilic to take a 5-4 lead on the Tualatin Hills hard court. Sock won the last four points of the final game, claiming the 3-hour, 13-minute match when Cilic hit his final shot into the net.
Sock said the victory is a career highlight.
''It's up there for sure,'' Sock said. ''Haven't played a ton of five-setters, haven't come back from two-down. To do that in front of the home crowd, it feels really special.''
The win over Cilic capped a big Friday for Sock, who earlier in the day was named to the U.S. Olympic team.
''As a kid you grow up and it's kind of on your bucket list. I'm very excited to get down there and hopefully play some good tennis,'' Sock said.
Isner rode the momentum of Sock's comeback win to the three-set win over the 19-year-old Coric.
''Jack had a lot of positive energy on his side. ... For me, it helps a lot. Took a lot of pressure off me. Regardless of what I do, we're not going to be losing. I was in a good spot, thanks to Jack,'' Isner said.
Isner, dominating with his big serve that reached as high as 137 mph, had 23 aces to Coric's 3. 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.
''I didn't use my chances in the first two sets. I had many chances to break him, and on 5-4 in both sets, I played a really poor game,'' Coric said.
The U.S. 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 United States is trying to break a nine-year Davis Cup title drought. The U.S. won in 2007 when it beat Russia 4-1 in Portland. Croatia claimed its only Davis Cup title in 2005.
The U.S. has won a Davis Cup-high 32 titles, but only one since 1995.
Should the United States clinch the Davis Cup tie Saturday, Sunday's reverse singles matches will be shortened to best-of-three sets.