"Today I'm not saying anything, like (Uruguay's Oscar) Tabarez (pictured)," Costa Rica's Brazilian coach Rene Simoes said.
"He's not talking either, he's not saying if he's going with three or four defenders."
Costa Rica are hoping to reach their fourth World Cup and third in a row, while twice winners Uruguay, who last played in the finals in 2002, are in their third successive play-off.
Simoes looks set to field three central defenders and two wing backs as he seeks an advantage to take to next Wednesday's second leg at the Centenario, where Uruguay won the first World Cup in 1930.
He will be looking to experienced defenders Gilberto Martinez of Brescia and home-based Luis Marin and Michael Umana to halt the expected counter-attacks spearheaded by Uruguay's Diego Forlan of Atletico Madrid and Luis Suarez of Ajax Amsterdam.
Simoes envisages two very different matches, fast on the synthetic surface at the Saprissa stadium in San Jose before a tougher, more physical game in Montevideo.
"We have to be strong, the two games will be different. We have to be calm. We want speed, intensity but we also have to be calm," he told the Costa Rican daily La Nacion.
Tabarez, whose team have been practicing on a synthetic surface in Guatemala before heading for Costa Rica on Friday, worked behind closed doors on Thursday and gave no news conference.
"It's going to be a very even series, very tough. We're prepared and dream of giving the Uruguayan people a happiness they deserve," captain Diego Lugano told reporters at the team's hotel in Guatemala City.
Costa Rica qualified for the two-legged playoff by finishing fourth in the CONCACAF's six-nation third round qualifying group. Uruguay were fifth in South America.
Uruguay met Australia in play-offs for the 2002 and 2006 World Cups, winning the first and losing the second.
When the two teams meet in Montevideo next Wednesday, due to the time zone, they will be vying for the 32nd and last berth at the finals in South Africa from June 11 to July 11, 2010.
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