CARSON, California, June 7 (Reuters) - A first-half goal from midfielder Clint Dempsey earned the United States a 1-0 victory over Guatemala in the opening Group B match at the CONCACAF Gold Cup on Thursday. Dempsey scored in the 26th minute at the Home Depot Center after Taylor Twellman's superb cross left him unmarked with only the goalkeeper to beat. It was Dempsey's eighth goal for his country in his 25th appearance. U.S. forward Landon Donovan was a constant threat on the right as the defending champions dominated the opening period before squandering two more opportunities shortly before halftime. The hosts survived an inspired performance by Guatemala in the second half when veteran playmaker Carlos Ruiz put Everton goalkeeper Tim Howard and his defence under constant pressure. Howard pulled off acrobatic saves in the 50th and 60th minutes and central defender Oguchi Onyewu was sent off in the 73rd after receiving his second yellow card. In the late Group B match, El Salvador came from behind to beat Trinidad & Tobago 2-1. Silvio Spann put Trinidad & Tobago ahead in the seventh minute with a curling free kick from 35 metres before Ramon Sanchez levelled half hour later. Dennis Alas sealed the win for El Salvador from close range in the 81st minute. Group B games continue on Saturday in Carson when Guatemala take on El Salvador and the U.S. play Trinidad & Tobago. The tournament shifts to New York on Friday when Panama face Honduras and Mexico meet Cuba in Group C. The CONCACAF Gold Cup, which will be played this month in six U.S. cities, will determine the champion of the region that includes North America, Central America and the Caribbean. The winning team will earn an automatic berth at the 2009 FIFA Confederations Cup.