MACAÍBA, Brazil—Cibele Florêncio was just 24 when she was crowned vice-champion at a national chess tournament in Brazil last year, no mean feat given she was spending up to 12 hours a day cleaning houses.

A maid and single mother from Brazil’s poor northeastern scrublands, Ms. Florêncio is competing at the highest level at what is still considered an elitist game in one of the world’s most unequal countries.

“People look at me and wonder what the hell I’m doing there,” she said of the other contenders at tournaments—players she described as mostly nerdy.

“But I use it to my advantage,” said Ms. Florêncio. “They let their guard down.”