The defending champion United States clinched the semifinal round of the Davis Cup today when Vitas Gerulaitis beat John Alexander of Australia, 5-7, 6-4, 8-6, 6-2, to give the Americans an unbeatable 3-1 lead.

The United States will oppose Italy in the Challenge Round in December. The Italians defeated Czecholsovakia, 4-1, in their semifinal Sunday.

Playing in windy conditions at White City, Gerulaitis recovered from a shaky start in which he dropped his service in the 12th game to give Alexander the set, 7-5. But Alexander could not handle Gerulaitis' return of service and dropped his serve in the first game of the second set. Gerulaitis, a finalist in the U.S. Open this year, dominated the rest of the match, taking the final set in 24 minutes.

A final singles match between John McEnroe of the U. S. and Mark Edmondson of Australia was to be played later today.

The Americans took a 2-0 lead when U.S. Open champion McEnroe defeated Alexander in the other singles match. Then the Aussies won the doubles Sunday, Alexander and Phil Dent combining to upend Stan Smith and Bob Lutz, 9-7, 6-4, 6-4. It was their first Davis Cup doubles loss since they were paired in 1968 for the first time.