Karin Knapp survived some shaky moments to defeat American Alison Riske 6-3 7-5 on Sunday and put defending champions Italy into the semi-finals of the Fed Cup.

Knapp’s triumph gave Italy an unbeatable 3-0 lead in the best-of-five match tie.

The Italian, ranked 40th in the world, had helped the visitors take a commanding 2-0 lead on Saturday with a 6-3 3-6 6-1 victory over Christina McHale in the opening match.

Camila Giorgi, ranked 84th, then downed 37th-ranked US teen Madison Keys 6-2 6-1, in only one hour in a matchup of Fed Cup debutantes.

In a desperate bid to keep the tie alive, US captain Mary Joe Fernandez opted to replace Keys in Sunday’s reverse singles, hoping that Riske’s more aggressive style would put more pressure on Knapp.

But Riske, playing her first live singles rubber in Fed Cup, had just six winners to Knapp’s 25 in the match lasting one hour and 37 minutes.

It could have been much shorter. Knapp served for the match at 5-2 and 5-4 in the second set and both times delivered nervous, error-strewn games.

Riske broke her to love to trim the gap to 5-3, then held her serve to get to 5-4.

She then broke the Italian again — getting plenty of help from Knapp’s two double-faults — to level the set at 5-5.

But Knapp broke Riske for the seventh time in the match to give herself another chance, and the Italian finished it out in style with a love game that included two great forehand winners followed by a service winner on her first match point.

The Italians next face either Spain or Czech Republic. Spain were 1-0 up in that tie in Seville, where the decisive matches were postponed until Monday because of rain.

Both Italy and the United States were without their top players for the tie on an indoor hard court in Cleveland.

Italy, seeking a fifth Fed Cup in nine years, were without Sara Errani and Flavia Pennetta, while the United States were without injured stars Serena Williams and Sloane Stephens.