MALAGA, Spain — Sara Errani stands at the baseline and exhales deeply. She is about to hit a second serve, with Italy up match point against Poland. A place in the Billie Jean King Cup final is at stake. So Errani does what she has done many, many times before: she hits an underarm serve. The ball floats into the service box and onto the racket of Iga Swiatek, one of two women’s players who can claim to be the best in the world. Swiatek is on to it in a flash and hits her return deep to Errani’s