Add in the U.S. Open title .
Kerber became only the second woman ever to win her first two Grand Slam singles titles after turning 28 , joining Li Na , who won the 2011 French Open at 29 and , three years later , the Australian Open .
Add in the U.S. Open title and Kerber became only the second woman ever to win her first two Grand Slam singles titles after turning 28 , joining Li Na , who won the 2011 French Open at 29 and , three years later , the Australian Open .