Serena Williams is among the investors of a $15-million Series A funding round to kickstart the digital expansion of startup, Let's Do This.

The Tennis superstar is joined by another star athlete, Usain Bolt, in the round of financing led by EQT ventures, which includes Y Combinator (a previous investor), NFX, Richard Oldfield, and Paddy Dear. Back in September, Williams and Bolt were also part of the seed round that raised $5 million for the startup.

According to Crunchbase, Let's Do This now has $21.4 in total funding.

The company is headquartered in San Fransisco and develops an online platform for listing races and endurance events in the U.S., the U.K., and Australia. They have over 30,000 listed races like half marathons, 10k runs, 5k runs, triathlons, and events like swimming, mountain biking, road cycling, and others.

Their platform can also recommend more personalized races by using algorithms provided by information from event history and other data. They earn revenue by sharing a percentage of ticket sales from race organizers.

Co-founders Sam Browne and Alex Rose, who both studied at the University of Cambridge, started Let's Do This with the intention of getting more people to exercise. Browne believes that races can help people adopt a fitness habit compared to other forms of physical activities since it encompasses SMART (specific, measurable, attainable, relevant, and time-bound) goals -- there's also social accountability.

“I’ve seen firsthand the incredible impact these events can have on making people fitter, healthier and happier. I love that Let’s Do This is not only making events like these more accessible but also helping to support athletes of all different fitness levels," commented Willaims.

"Women are especially less likely to participate in marathons and obstacle races, so it’s really important there’s a platform encouraging people to step out of their comfort zones and make a positive difference in their lives."

This is not the only startup that received William's backing recently. Williams also partnered with other athletes like Kevin Durant and Larry Fitzgerald last month to finance Mercury, a digital bank for startups. She also invested in CoinTracker, a company that allows people to calculate taxes on crypto gains.

Serena Williams powered into a 38th Grand Slam semi-final at the US Open on Tuesday