US women's soccer star Sophia Smith celebrated her World Cup goal with the zip of her lips. Here's the heart-wrenching backstory behind the move.

• Sophia Smith scored the US Women's National Team's first two goals of the 2023 World Cup.
• The star forward celebrated her second goal by honoring her late friend and teammate, Katie Meyer.

With two goals and an assist in the US Women's National Team's World Cup opener against Vietnam, Sophia Smith is already becoming the reigning champions' breakout star of the tournament.

But for the 22-year-old forward, this summer's campaign Down Under is about much more than scoring goals and winning games.

Smith and Naomi Girma, a center-back for USWNT, have dedicated this World Cup to their late best friend and college teammate, Katie Meyer. More than 1 ½ years after the former Stanford Cardinal goalkeeper died by suicide on campus in Palo Alto, California, Smith and Girma spearheaded the team's efforts to destigmatize mental illness through their partnership with "Common Goal."

"Since then, I've had a different perspective on life and on people," Smith told NBC of Meyer's death. "I try to approach everything with like, you don't know what someone's going through, and you won't always see signs of struggle."

On Friday, Smith celebrated scoring her second goal of the day in her first-ever World Cup showing by paying homage to Meyer. Once the Video Assistant Referee review confirmed that her goal counted, Smith embraced her teammates, then mimed zipping her lips and throwing away the key.

When Meyer saved two crucial penalty kicks to guide Stanford to the 2019 national championship, she excitedly celebrated in kind. Check out the two celebrations side by side:

After her team's 3-0 win against Vietnam, Smith told reporters, including The Athletic's Meg Linehan, that her second goal celebration "was for Katie."

"That was pretty iconic what she did in the College Cup, and we just want to honor her in every way," Smith said.

During her press conference Monday, Girma said that she and Smith had agreed to pull out "Katie's go-to" celebration "if one of us scored — probably her." Ahead of the tournament, Girma wrote a heartfelt essay for The Players' Tribune titled "This Is For Katie," in which she discussed her friend and the importance of talking about mental illness.

"It's just another way of us honoring her, like The Players' Tribune piece and a lot of players just speaking out on mental health," she added in the press conference. "We see this as an opportunity to shed light on a lot of the things that are important to us, and I think that's been something that's at the core of this team for so long. For us to come in now and carry on that legacy is something that's really important to us."

The US Women's National Soccer Team is set to return to the pitch Wednesday at 9 p.m. ET to take on the Netherlands — their 2019 World Cup final foe — in Wellington, New Zealand. Fans stateside can tune into the action on FOX.