Serena Williams retains WTA Finals title . Beats Simona Halep 6 - 1 6 - 1 in semifinal match in Singapore . Halep thrashed Williams in round robin match earlier in week . Fifth place in season - ending championships for Serena Williams .
Serena Williams retains WTA Finals title . Beats Simona Halep 6-3 6-0 in title match in Singapore . Halep thrashed Williams in round robin match earlier in week . Fifth victory in season-ending championships for No.1 Williams .