A Serena Williams-Li Na semifinal will be a mental test for both

Throughout this U.S. Open Williams has remained poised and focused, staring down opponents as if she were a world-class heavyweight champ looking to defend

Li Na fell in straight sets to Serena Williams earlier this year at the West and Southern Open in Cincinnati :: Matthew Stockman/Getty Images

Throughout this U.S. Open Williams has remained poised and focused, staring down opponents as if she were a world-class heavyweight champ looking to defend her title. Come to think of it, that's exactly what she is.