Five members of the U.S. Greco-Roman wrestling team were unbeaten after the first day of competition at the Pan American Wrestling Championships in Caracas, Venezuela, March 14.    Held three of every four years during the non-Pan American Games years, the Pan American Championships features the best wrestlers in North, Central and South America.     U.S. wrestlers with three wins through the first day were Mike Ellsworth (Colorado Springs, Colo./Michigan WC) at 66 kg/145.5 lbs. and Keith Sieracki (Colorado Springs, Colo./U.S. Army) at 74 kg/163 lbs.     Ellsworth scored two pins and a tight 6-5 win over Enrique Cubas of Peru.    Sieracki, a member of the 2001 U.S. World Team, scored a pin, a technical fall and an 8-1 decision over veteran Cuban star Cecilio Rodriguez.    Wrestlers with two wins on the first day included Jim Gruenwald (Colorado Springs, Colo./Sunkist Kids) at 60 kg/132 lbs. and Dremiel Byers (Colorado Springs, Colo./U.S. Army) at 120 kg/264.5 lbs.    Gruenwald, a 2000 Olympian, scored a pair of 10-0 technical falls, beating Ron Bocher of Canada and Julio C. Cueno of Colombia.    Byers, who was sixth in the 1999 World Championships, scored a technical fall in his first match, then stopped Guillermo Talavera of Venezuela, 8-0.    Winning his only match of the day was Ethan Bosch (Colorado Springs, Colo./New York AC), who pinned Olguin Sabua of the Dominican Republic in 2:25 at 84 kg/185 lbs.    Jeff Cervone (Colorado Springs, Colo./U.S. Air Force) had an 0-1 record at 55 kg/121 lbs. and Ross Thatcher (State College, Pa./New York AC) had a 1-1 record at 96 kg/211.45 lbs.    PAN AMERICAN CHAMPIONSHIPS  at Caracas, Venezuela, March 14  U.S. performances  55 kg/121 lbs. - Jeff Cervone, Colorado Springs, Colo. (U.S. Air Force) - lost dec. to Jose Cordova (Venezuela), 7-0    60 kg/132 lbs. -Jim Gruenwald, Colorado Springs, Colo. (Sunkist Kids) - won by tech. fall over Ron Bocher (Canada), 10-0, 2:55; won by tech. fall over Julio C. Cuenu (Colombia), 10-0, 2:00    66 kg/145.5 lbs. - Mike Ellsworth, Colorado Springs, Colo. (Michigan WC) - dec. Enrique Cubas (Peru), 6-5; pin Mario Valdiva (Chile), 2:28; won by pin in 2:06    74 kg/163 lbs. - Keith Sieracki, Colorado Springs, Colo. (U.S. Army) - won by tech. fall over Sixto Barrera (Peru), 10-0, 1:38; pin Paul Perez (Venezuela), 2:40; dec. Cecilio Rodriguez (Cuba), 8-1    84 kg/185 lbs. -  Ethan Bosch, Colorado Springs, Colo. (New York AC) - pin Olguin Sabua (Dominican Republic), 2:25    96 kg/211.45 lbs. - Ross Thatcher, State College, Pa. (New York AC) - lost pin to Pena (Cuba), 3:02; pin Gabriel Willson (Dominican Republic), 2:24    120 kg/264.5 lbs. - Dremiel Byers, Colorado Springs, Colo. (U.S. Army) - won by tech. fall over (Peru), 10-0, 3:08; dec. Guillermo Talavera (Venezuela), 8-0