NASHVILLE — The United States is off to a perfect start in Davis Cup play this year, and the Americans now have to wait and see if they can continue this momentum when they play in the semifinals for the first time since 2012.

Ryan Harrison had the honours Sunday wrapping up a 4-0 victory over Belgium in essentially an exhibition match with the U.S. already clinching a semifinal berth against Croatia on Sept. 14-16. Harrison, ranked 54th in the world in singles, beat Ruben Bemelmans 6-3, 6-4 in 65 minutes after the U.S. won the first three matches to improve to 5-0 all-time in Davis Cup play against Belgium.

"It's one of the great challenges in Davis Cup in this current format is the momentum disappears because you don't play again for months on end, so you have to recreate that momentum," U.S. captain Jim Courier said. "As we get going toward the next time in September, it comes on the heels of the U.S. Open, so we'll be leaving after the U.S. Open to go over to Croatia."

That's a long time between matches with three Grand Slams between now and then leaving open the possibility of injuries, which could affect the current roster of John Isner, Sam Querrey, Jack Sock, Steve Johnson and Harrison. Johnson was the only American not to play in the quarterfinals at Belmont University. Courier said he knows all of the players want to be in Croatia.

"Hopefully, they'll be healthy and playing great, and we'll have the same crew because the energy and camaraderie is really good with the guys and the options that we have in singles and doubles are really strong and a big benefit," Courier said.

Courier doesn't see any issue with the Americans becoming overconfident, not when they know too well that both Serbia and Belgium were missing key players. Steve Darcis and David Goffin both were out for Belgium, which had reached two of the last three Davis Cup finals.

"I think we all know that we still have a lot to prove," Courier said. "We'll be going on the road to play a very deep team in Croatia, a team that beat us two years ago in the quarterfinals when we were up 2-0. They came back and won the last three matches. So we'll have plenty on our minds when we go there."

Croatia advanced Sunday rather easily with Marin Cilic downing Mikhail Kukushkin 6-1, 6-1, 6-1.

Spain needed all five matches to advance 3-2 to a semifinal against France. Rafael Nadal started the rally by beating Alexander Zverev 6-1, 6-4, 6-4, and David Ferrer clinched the quarterfinal for the Spaniards downing Philipp Kohlschreiber 7-6 (1), 3-6, 7-6 (4), 4-6, 7-5. France advanced with a 3-1 win over Italy as Lucas Pouille defeated Fabio Fognini 2-6, 6-1, 7-6 (3), 6-3.

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