Venus and Serena Williams champion women's rights

Venus Williams used her signature long reach to stretch for the ball, her racket just grazing it as it thundered past her. The crowd cheered wildly as her defiant opponent battled back, forcing a 12-shot rally in the blistering heat.

Eventually the seven-time Grand Slam winner clinched the game. Her challenger, nine-year-old Lola Holmes, was nevertheless thrilled.

"Venus is my favourite player. I was just thinking, wow, how am I even doing this when I was hitting the ball back. It was amazing," Holmes said breathlessly afterwards, clutching her racket and grinning from ear to ear.

She was one of several dozen young children in Nigeria being coached for an afternoon on Wednesday by two of modern tennis' biggest stars. Venus and Serena Williams kicked off an "Africa tour" in promotion of women's rights that will take in Nigeria before moving on to South Africa.

Hundreds packed the upmarket club in Lagos for the tennis clinic, braving the relentless heat as they squeezed into all available spaces. As the sisters, wearing bright pink and orange tops, tossed balls on the neat centre court, office workers hung out the windows of overlooking tower blocks and cheered the budding tennis stars.

"We're here to empower young girls and let them know that if you dare to dream, you can achieve any goal you want to," Serena said at an earlier press conference held by the Breaking The Mould initiative they are representing.

The initiative is aimed at encouraging "more women to break moulds that have stood between them and their potential," the organisation said in a statement. There are 80 million women in Nigeria, but gender disparity is an acute problem. Africa's most populous country ranks 118 out of 134 countries on the Gender Equality Index, according to a British Council study.

"We were able to break the mould when tennis was very dominated by white people. To have a face of colour come in and dominate showed it doesn't matter what your background is and where you come from, if you have dreams and goals, that's all that matters," Serena said.

Looking visibly tired two days after beating Maria Sharapova to win the women's world tennis championships, she said the trip was special in other ways too.

Lagos was the first African city she and her sisters had seen during a plane stopover in 1998, she recalled. "We looked out the window and we were like, we finally made it to Africa. We definitely would love to see more athletes come out of Africa," she added.

But she politely brushed off attempts to claim her as an African superstar, with one particularly enthusiastic journalist suggesting the specific Nigerian town where their ancestors may have been from. "I'd love to say I'm from Africa, but we're from the United States and we're proud to be from there as well," Serena said.

Venus said a planned exhibition match on Friday – their first match in Africa – would be a highlight. "After all the finals everywhere … this will be our final in Nigeria," she said to cheers. "I don't know who's going to win but Serena just won the championship so I know she's ready. I'll have to play my best tennis."

At the lively press conference, the two sisters, who between them hold 22 major singles titles, also faced eager questions from a ten-year-old girl, Zuriah Oluwale, who quizzed Serena on whether she planned to fund any schools in Nigeria as she had done in Kenya in 2008. "I think it would be a good plan for the future," Serena said

"My next two questions are for Venus," Oluwale said, to which Venus jokingly quipped: "I'm ready."

The question – how Venus felt winning her first Wimbledon Grand Slam in 2000, two years before her questioner was born – threw into spotlight how far the sisters' have changed the game of tennis.

"Nigerians love tennis but there has been a lack of Nigerian role models since the early 1990s. It's possible some mothers saw the Williams playing, saw two black women being successful on the court, and maybe that made it easier for us as mothers to think our girls can play tennis seriously too," said Lagos resident Ebifegha Netimah, whose two teenage daughters "want to be Venus and Serena".

While the theme of the tour was empowering girls, the Williams sisters appeal to young players of both genders.

Seventeen-year-old Andrew learnt to play tennis after taking up work as a ballboy in an upmarket club. "I used to go very early or late at night before anybody else was there and hit the ball with the other ball boys. We didn't have money so we used rackets from the sports shop, and then put them back before anybody caught us."

"I really like the Williams sisters. This week I was so excited they were coming, I forgot the tiredness, I just wanted to see them because I know it will encourage me to keep playing," said Andrew.