Petri began playing water polo on the boys' team in high school, but helped begin a girls' water polo program at Miramonte High School and was team captain for two years. In 1997 she was recruited by UC Berkeley coach Maureen O'Toole, and played collegiate water polo from 1997-2001. Petri earned All-America and All-MPSF honors at Cal in 1998 and 1999 and finished her college career with 96 goals. She sat out her senior season as a member of the first US women's Olympic water polo team, and graduated from Berkeley in 2002 with a degree in integrative biology. She played for Rari-Nantes, a professional water polo team in Florence, Italy, for the 2004-2005 and 2005-2006 season, and is currently a member of the US senior women's team.
In March 2007, Petri competed in the 2007 World Aquatics Championships, representing the United States. After a series of victories, the United States won the gold medal, becoming world champions. Petri also won a gold medal in the 2003 World Aquatic Championships in Barcelona, Spain and in 2009 in Rome. She is now a three time World Champion.
At the 2008 China Summer Olympic games, she and the American team lost 8-9 in the Championship game to the Netherlands and took home the silver medal.