Kerber ended Serena 's 186 - week reign as the No. 1 player in the world at the United States Open last September when Kerber won in the semifinals .
Williams was stunned in the same round by Czech player Karolina Pliskova .
Kerber ended Serena 's 186 - week reign as the No. 1 player in the world at the United States Open last September when Kerber won in the semifinals and Williams was stunned in the same round by Czech player Karolina Pliskova .