Serena Williams ' shock defeat less than 40 minutes before Kerber 's match in Arthur Ashe Stadium sealed the German 's place as World No 1 .
Kerber underlined Kerber 's credentials against Wozniacki , winning 6 - 4 , 6 - 3 .
Serena Williams ' shock defeat less than 40 minutes before Kerber 's match in Arthur Ashe Stadium sealed the German 's place as World No 1 and she underlined her credentials against Wozniacki , winning 6 - 4 , 6 - 3 .