Tseng turned professional in January 2007. That year she competed on the Ladies Asian Golf Tour and won the DLF Women’s Indian Open. She also competed on the CN Canadian Women’s Tour where she won the CN Canadian Women’s Tour at Vancouver Golf Club. In January 2011, Tseng defended her title at the Taifong Ladies Open on the LPGA of Taiwan Tour. Three weeks later she won the ISPS Handa Women's Australian Open and a week later the ANZ RACV Ladies Masters, both events co-sponsored by the ALPG Tour and the Ladies European Tour. Her wins moved her into the number 1 position in the Women's World Golf Rankings. She won again the next week in the first tournament of the LPGA season, the Honda LPGA Thailand. In June 2011, she won the LPGA State Farm Classic over Cristie Kerr by three strokes. Two weeks later, she won the LPGA Championship. This made her the youngest player to win four LPGA majors. The next month she defended her title at the Women's British Open, becoming the first defending champion winner at the Women's British Open as a major. Her five major titles also made her the youngest player, male or female, to win five major championships. Tseng won the LPGA Player of the Year for a second straight year. She wrapped up the award while the season still had four events remaining.