(CNN) When most people think about domestic violence, they probably think of physical, sexual or emotional abuse, but less well-known is how partners can use financial means to abuse and terrorize their partners.
Tennis superstar Serena Williams hopes to pull the issue out of the shadows. By speaking out and adding her name to the cause, she's trying to make more people aware of the perils of economic abuse and realize what they can do to help victims.
"While I was familiar with financial abuse, I really didn't realize that it happened in about 99% of domestic violence cases. That's a pretty insane number. That's basically every domestic violence case," Williams said. "I was really surprised at how prevalent it was and underexposed the issue of financial abuse was. Because of that, I kind of wanted to encourage others to stand up and speak up and speak out about financial abuse."
Economic abuse can take many forms: preventing a victim from attending a job or looking for a job, or harassing a victim while on the job; applying for credit cards in a victim's name without their consent and running up mountains of debt; and deciding when or how a victim can have access to cash or credit cards.
For a sense of how widespread the problem is, as Williams mentioned, nearly all domestic violence survivors -- between 94% and 99% of them -- also report some form of economic abuse, according to the National Coalition Against Domestic Violence.
Victims of domestic violence lose eight million days of paid work each year, the coalition says. The cost of the abuse is more than $8 billion per year.
One in four women will probably be affected by domestic violence at some point in their lives, and finances are often one of the top reasons women can't leave an abusive relationship, experts say.
"There are so many ways that financial abuse can keep their victims trapped," said Williams, who is the new ambassador for the Allstate Foundation Purple Purse program. Purple Purse, founded in 2005, focuses on raising awareness and helping survivors find the tools they need to recover from financial abuse.
"If a woman's credit is ruined, she can't go out and move out and try and get a different apartment. She has to kind of stay where she is," Williams said.
Vicky Dinges, the senior vice president for corporate responsibility for Allstate, said victims face economic hardships even if they find a way to leave the relationship.
"One of those things that we see when a victim does leave safely, they have a hard time affording legal fees, trying to rent an apartment, trying to see their children if they have children," Dinges said.
Nearly 80% of Americans, according to a 2014 survey by Allstate, said they didn't know much about how financial abuse was a form of domestic violence. That lack of understanding is reflected in a new public service announcement from the Purple Purse campaign.
In front of a hidden camera, passengers get into a Lyft car and come across a purple purse. Inside, they find a phone and see a series of abusive text messages.