WATCH: Marino last played Williams at the 2010 US Open, and made an epic comeback on Monday night against the former world No. 1.

WASHINGTON—Venus Williams only just began her comeback earlier this summer, but Rebecca Marino has been on the trail far longer, and on Monday night the Canadian qualifier earned one of her biggest milestones yet with a 4-6, 6-1, 6-4 victory over the former world No. 1 at the Citi Open.

Playing the five-time Wimbledon champion for the first time in over a decade, Marino avenged the defeat in epic fashion: rallying from both a set down and 4-1 in the deciding set to advance in just under two hours.

Williams drew a sold-out crowd to Stadium Court ahead of what would be her first singles match since last August, when she bowed out of a small Chicago tournament just before the US Open. Williams made a surprise comeback in mixed doubles last month at Wimbledon, winning a round with another famous elder sibling Jamie Murray and, with little sister Serena watching in the stands, looked strong to start her night session encounter with Marino.

The two played once before at the 2010 US Open, when a then-No. 3-ranked Venus remarked just how similarly the two big-hitters played.

“I guess I know what it's like now playing myself,” she said at the time.

Marino looked to be a star of the future, breaking into the Top 40 the following season, but persistent issues with anxiety and depression forced her off tour for much of the next five years.

Making a tentative return to action in 2018, Marino has slowly moved up the rankings and shown flashes of brilliance particularly in the last year, particularly at home when she defeated Paula Badosa in three sets at the National Bank Open.

She made long-awaited returns to the Roland Garros and Wimbledon main draws earlier this summer, but it was all building towards this full-circle moment against Venus, who played a set and a half of brilliant tennis but couldn’t hold off an inspired Marino in the final five games.

"It was so much fun to play in D.C.," a smiling Venus said in defeat. "It was nice to have the crowd behind me. Haven't played a singles match in a year, so definitely a great experience.

"It's my first match, so I didn't think I played well a lot of the times. Just a little rusty. I really put myself in good positions in the third set, too. Just trying to shake off some rust. That's just to be expected. All I can do is just play another tournament and play better."

With No. 8 seed Clara Tauson or Andrea Petkovic standing between her and the Citi Open quarterfinals, Marino can likely move back into the Top 100 after enjoying a brief stint at No. 99 after Wimbledon.

The strong serving and clean ball-striking that so impressed Venus in 2010 were on full display in D.C.; should she replicate this form in subsequent matches, Marino may yet be the player she might have been.