From designing her on-court outfits to launching her own off-court clothing line, fashion has always been central to Serena Williams' interests away from tennis -- and on Thursday night, the 23-time Grand Slam champion passed on her wisdom to the four finalists on the latest season of Bravo's Project Runway.

Williams joined judges Brandon Maxwell, Nina Garcia and Elaine Welteroth, host Karlie Kloss, and mentor Christian Siriano, for part two of the season 18 finale, where Williams was introduced as not only "the greatest tennis player of all time," but an "all-around revolutionary woman."

Designers Victoria Cocieru, Sergio Guadarrama, Geoffrey Mac and Nancy Volpe-Beringer displayed their final collections at New York Fashion Week, and in her appearance, Williams was full of praise for Mac, the eventual winner, who was candid about his struggles of finding his self-belief over the course of his time on the show.

"It's so important to believe in yourself," she told him. "In my job, I look different, and that kind of put me on this path of being different.

"You have a lot to say and to design, and you should just be yourself." 

In PEOPLE magazine, the WTA legend also noted her appreciation for Volpe-Beringer's collection, which highlighted the larger trend of sustainability in fashion.

“We have to somehow figure out how to live on this planet and not destroy it, so this really speaks to me personally," Williams said of the collection, and the efforts for sustainability as a whole.

"It has to be incorporated into design as well because fashion is a really big part of a lot of bad things that are happening and it’s just the honest truth.

"It’s about how do you create those collections that are more sustainable and takes less pressure from what we do.”

The appearance on the competition show was the latest fashion venture for the 23-time Grand Slam champion this year, as she presented the spring 2020 collection for her S by Serena clothing line at New York Fashion Week in February. 

She appeared at an event with Vogue editor-in-chief and longtime tennis fan Anna Wintour, and also revealed her thoughts pursuing fashion design to PEOPLE.

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"You grow up looking up to people like Anna and so many amazing fashion designers, and now, you’re just in the room making decisions and it’s different,” she said.

“You grow and you’re doing what you want to do, and I think this is just us reaching our goals.”