The story reads like a tall tale, no matter how many times it’s repeated – too incredible to be true and growing only more preposterous with each passing year.

A young father of three in Compton stumbles upon a tennis match one Sunday while channel-surfing, struck by astonishment as the winner, a Romanian player named Virginia Ruzici, is presented with a $40,000 check. He decides right then that his next two daughters – who, it should be noted, were still unborn – will be professional tennis players.

With 10¢ balls taken from grocery trolleys, the son of a cotton picker, who dropped out of high school and has a tennis resume as long as a postage stamp, drills his charges on public courts that are cracked. How they chuckle when he says his daughters—who aren’t even teens yet—will play each other in the Grand Slam finals one day. Venus is going to be the world’s best player. He promises that Serena will be even better.

Nearly four decades after he first spotted opportunity in the dim flicker of the tube, all of Richard Williams’ most quixotic predictions have come to pass. If anything he undershot them. This ultimate American folk tale remains a work in progress, the latest instalment forthcoming with Saturday at Melbourne Park when Venus and Serena – the sisters with 29 major singles titles, eight Olympic gold medals and more than $100m in prize money between them – meet once more for the Australian Open title.

Venus-Serena XXVIII marks the ninth time the Williams sisters have faced off in a grand slam final and their 15th meeting at one of the sport’s four bedrock events. Serena holds a 16-11 advantage in the all-time series with a 9-5 edge at majors. That Saturday’s showdown will take place on the court where they played their first professional match against one another – a second-rounder at the 1998 Australian Open – lends a touch of symmetry to the proceedings.

Venus, who is 36 and the oldest woman in the top 300, is aiming for her eighth grand slam title and first since Wimbledon in 2008, when she toppled her sister in the final. A victory would make her the oldest player to win a major singles championship in the Open era, breaking the mark set by Serena at the All England Club last year. It’s her first final in Melbourne in 14 years.

The winter of the elder Williams’ career, too often dwarfed by Serena’s incandescent third act, has been nearly as remarkable. She is playing in her 73rd major tournament – most in the Open era – and finds herself back in the top 20 alongside players nearly half her age. That her return to the brink of a major title comes six years after she was diagnosed with Sjögren’s syndrome, an incurable autoimmune disease with unpredictable symptoms that cast major doubt on her tennis future, is all the more stunning.

“When I’m playing on the court with her, I think I’m playing the best competitor in the game,” Venus, the No13 seed, said after Thursday’s fightback win over CoCo Vandeweghe in the semis. “I don’t think I’m chump change either, you know. I can compete against any odds. No matter what, I get out there and I compete.

“So it’s like two players who really, really can compete, then also they can play tennis. Then, okay, won’t be an easy match. It’s like I know that it won’t be easy. You have to control yourself, then you also have to hopefully put your opponent in a box. This opponent is your sister, and she’s super awesome.”

Serena, 35, is targeting her 23rd major title to move past Steffi Graf for most in the Open era. It would also propel the world No2 back into the top spot after ceding the mantle to Angelique Kerber last fall, making her the oldest No1 since the WTA introduced computerized rankings in 1975. She’s surged to the final without dropping a set.

“I just feel like it’s been a while,” the second-seeded Serena said on Thursday of facing her sister in the final. “This probably is the moment of our careers so far. I can definitely say for me.

“I never lost hope of us being able to play each other in a final, although it was hard because we’re usually on the same side of the draw. Whenever we’re on the opposite side, I always definitely feel a lot better.”

The match offers a showcase of the sisters’ extraordinary longevity. It’s only the third grand slam final of the Open era to feature two women in their 30s – and their combined age of 71 makes it the oldest major final by more than five years.

The oddsmakers have instaled Serena as the favorite on merit, but the curious symbiosis at play lends a tinge of unpredictability to the proceedings. Venus, who knows Serena’s game better than anyone else, has more wins over her sister than any opponent past or present. Their most recent meeting – Serena’s three-set win in the 2015 US Open quarter-finals – could have gone either way.

The only certainty a 30th grand slam title will be added to the family ledger as a father’s vision is further delivered.

“I just feel like my dad really had some innovative things that we worked on and that we always played,” Serena said. “He taught us different techniques that no one else was doing. People were like, What are you doing? That’s not the right way.

“But my dad just knew, watched footwork, knew different things that would work better for tennis. He taught it to Venus and I. We definitely were able to revolutionize a lot of things in the game. We were able to come out here and just be two really strong women standing up for everything we believe in.”