Proof That Bethanie Mattek-Sands Is Serena's Most Eccentric Obstacle

Serena Williams, the reigning champ of every Grand Slam event and five wins away from becoming the first tennis player since 1988 to win the Grand Slam, faces fellow American Bethanie Mattek-Sands on Friday at the US Open.

So just who is Mattek-Sands? Despite this being her 13th appearance at Flushing Meadows, the 30-year-old American is still a relative unknown, even in her own country. Here are five things to know about the unheralded star.

1. She's perhaps best known in the tennis community for her ever-changing hair color and unique sense of style. Currently rocking orange hair (she's sported everything from unicorn tips to a bright blue-and-green 'do), the Phoenix resident is rarely without her signature knee-high compression socks on the court.

The socks are frequently paired with other, let's just say, unconventional tennis outfits.

2. She's a four-time Grand Slam champion. In doubles, that is. Mattek-Sands and her pal Lucie Safarova teamed up shortly before the 2015 Australian Open, and not only did they win the title there, but they went on to win at the French Open as well. She's now the No. 3 ranked doubles player in the world. Most recently the winning pair -- nicknamed #TeamBucie -- nabbed the title at the Rogers Cup in Toronto.

She also has won two Grand Slam mixed doubles titles, emerging victorious at the 2012 Australian Open with Horia Tecău and the 2015 French Open with Mike Bryan.

3. She's besties with fellow tennis players Sania Mirza and Monica Puig. Luckily for us, that means she is frequently posting hilarious videos and photos when she's with either of them.

4. She and husband Justin Sands have a 140-pound Boerboel mastiff named Ruger, who is the subject of many, many of her posts on social media. Because, duh, look at this adorable guy.

5. She's the ultimate anti-establishment hero. Despite tennis' reputation as a "gentleman's game," Mattek-Sands marches to beat of her own drum. From pushing the limits to the dress code at Wimbledon...

...To sporting a killer sleeve on her arm, she's unlike just about anyone in tennis.