Pierce said that with Williams serving and returning as well as she is at the moment, no other player stands a chance of stopping her in the tournament.
"Serena is just too strong, she's on another level," BBC Sport quotes Pierce as saying. "She hits harder, she moves faster than anyone.
"There is nobody in my opinion that can beat Serena here this year. Her serves and returns are the key points. She's returning really well and she did a great job tonight."