(Reuters) - Americans Jack Sock and Ryan Harrison rallied from a set down in a tight doubles rubber to ensure the United States became the first team to advance to the Davis Cup semi-finals on Saturday with a 3-0 victory over Belgium in Nashville, Tennessee.

April 7, 2018; Nashville, TN, USA; John Isner (USA) and Sam Querrey (USA) and Jack Sock (USA) and Ryan Harrison (USA) and Steve Johnson (USA) celebrate winning the Davis Cup tie against Belgium at Belmont University. Mandatory Credit: Susan Mullane-USA TODAY Sports

Sock and Harrison failed to break serve until the fourth set yet managed to see off doubles specialists Sander Gille and Joran Vliegen 5-7 7-6(1) 7-6(3) 6-4 to wrap up the tie in the fewest possible matches for the Americans.

The U.S. duo overcame some nervous moments but were able to win key points in the two tiebreakers to secure a victory to ensure the record 32-times winners advanced to the last four.

“It wasn’t easy, the Belgians certainly played their part as well,” U.S. captain Jim Courier said.

“This (Belgian) doubles team is not one that we knew a whole lot about, but they earned a lot of respect from our guys. Took us a while to really dig our teeth into the match and that second set tiebreaker was crucial for us.”

In the opening set, Sock’s serve was broken in the 11th game as the Belgians forged an early advantage in a match they needed to win to retain any hope of reaching a third straight Davis Cup final.

The situation appeared even bleaker for the Americans in the second set when they fell 15-40 down while serving at 5-5, but this time Sock staved off two break points and the hosts won the ensuing tiebreaker and never looked back.

“They came out with a lot of energy and were really, really on top of the net,” Harrison said.

“We were having a tough time settling down and settling into a rhythm because the points were really short. We ended up winning a pretty comfortable breaker and from that I felt like we were pretty much on top and we just had to finish it off.”

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The United States were set on their way to victory when John Isner and Sam Querrey won their opening singles matches on Friday and they will face either Croatia or Kazakhstan in the semi-finals in September.

SPAIN SHOCKED

In other quarter-final ties, Germany’s Jan-Lennard Struff and Tim Puetz pulled off a shock 6-3 6-4 3-6 6-7(4) 7-5 win over Spain’s Feliciano Lopez and Marc Lopez to give Germany a 2-1 lead in Valencia.

Germany raced into a two-set lead but with world number one Rafael Nadal roaring on his team mates, Spain fought back to force a fifth set at a packed Plaza de Toros arena.

Spain squandered five break points in the eighth game of the decider and they ultimately paid the price when Marc Lopez, drafted in to replace an injured Pablo Carreno Busta, was broken three games later.

Germany pounced on their chance and served out to win the marathon in four hours and 23 minutes.

On Sunday, world number four Alexander Zverev could secure Germany’s spot in the semi-finals if he can snap Nadal’s record 23-match winning streak in the men’s team competition.

David Ferrer will play either Philipp Kohlschreiber or Struff in the final rubber.

Holders France took a 2-1 lead in their tie with Italy as Pierre-Hugues Herbert and Nicolas Mahut comfortably beat Simone Bolelli and Fabio Fognini 6-4 6-3 6-1.

In Varazdin, Ivan Dodig and Nikola Mektic overcame Kazakhstan’s Timur Khabibulin and Aleksandr Nedovyesov 6-7(2) 6-4 6-4 6-2 to hand hosts Croatia a 2-1 lead.