'Mom doesn't know about it!' Marta Kostyuk spills the beans on little sister Zoryana's first 💅

WATCH: Before beating Caroline Garcia, Marta Kostyuk saved match points in her first-round win over Bianca Andreescu.

Little sister, you're on notice! Zoryana Kostyuk is reaping the rewards of being her big sister Marta Kostyuk's lucky charm this week at the Mubadala Citi DC Open.

The elder Kostyuk is through to the quarterfinals at the WTA 500 event, after saving match points in a dramatic first-round win over 2019 US Open champion Bianca Andreescu, and earning a Top 10 win over No. 2 seed Caroline Garcia in the second round.

Asked by on-court emcee Rennae Stubbs if the youngster deserved a "bonus" for being her coach this week, Marta Kostyuk revealed that she's already making good on her side of the bargain.

"She definitely already got some bonuses," she said, laughing. "She did her nails for the first time in her life, so I think it's a good bonus to start with, and mom doesn't know about it, so it's like a little secret, too!"

Spotted on camera, the younger Kostyuk, eight years her sister's junior, couldn't contain her laugher, and showed off her perfect French tips.

Zoryana Kostyuk is, in fact, a talented young tennis player herself. She frequently accompanies her older sister on tour, and last summer, had the opportunity to have a hit with Fabio Fognini at the Western & Southern Open in Cincinnati.

On their exploits together this week in the U.S. capital, Marta Kostyuk said, "She's a great support and I'm very happy to share this week with her."

The champion in Austin in March, the Ukrainian will next face defending champion Liudmila Samsonova for a spot in the semifinals.