(Reuters) - List of U.S. Open women’s singles champions since tennis turned professional in 1968: 2019 - Bianca Andreescu (Canada) beat Serena Williams (U.S.) 6-3 7-5 2018 - Naomi Osaka (Japan) beat Serena Williams (U.S.) 6-2 6-4 2017 - Sloane Stephens (U.S.) beat Madison Keys (U.S.) 6-3 6-0 2016 - Angelique Kerber (Germany) beat Karolina…