Venus and Serena Williams, at 38 and 37 years old, have achieved more in their careers than most people could dream of: Between the two of them, they've won 30 Grand Slam singles titles (that's in addition to the doubles titles they've won together), and they've earned $130 million in prize money alone.

What's more, they both have their own fashion lines and have been minority owners of the Miami Dolphins since 2009. They were the first female African Americans to hold an ownership stake in an NFL franchise.

But if you ask them, the tennis legends will say they haven't yet "made it."

"I don't think like that," Serena told CNBC in 2016. "I'm still going and doing the best that I can."

Her older sister said something similar in a 2018 interview with CNBC Make It: "I'm always trying to make it. It's not about getting there. It's about what you achieve every single day, so in that sense I don't ever feel like I've made it."

Perhaps the fact that they don't rest on their laurels is part of the reason they’ve continued to achieve. According to Richard St. John, who interviewed hundreds of impressive individuals for his book "The 8 Traits Successful People Have in Common," the world's highest achievers are never complacent.

Instead, he writes, they continually push themselves out of their comfort zones and are always improving, "whether it's their career, project, product or service."

The Williams sisters have been competing on the pro tour for more than two decades, and they show no signs of slowing down. They're both through to the third round of the 2019 Australian Open and could collide in the fourth round if they both win their next matches.

Serena, seeded No. 16, plays Dayana Yastremska, while Venus takes on No. 1 seed Simona Halep. Both matches are slated to take place on Saturday, January 19.

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