Serena Williams is calling out the way women are treated in the sporting world.

Tennis champion Serena Williams has called out gender inequality in the sporting world and encouraged girls to “dream big” in an open letter.

The seven-time Wimbledon winner challenged labels like “female athlete” in a letter published in Porter Magazine’s Incredible Women of 2016 issue.

Addressing the letter to “all incredible women who strive for excellence”, Williams opened up about the barriers she and other women athletes had faced in their paths to success.

“One of those barriers is the way we are constantly reminded we are not men, as if it is a flaw,” she wrote.

“People call me one of the “world’s greatest female athletes”.”

A photo posted by Serena Williams (@serenawilliams) on Nov 5, 2016 at 6:14am PDT

“Do they say LeBron is one of the world’s best male athletes? Is Tiger? Federer? Why not? They are certainly not female.”

The 35-year-old, who has called out sexism throughout her career, urged readers to not let this kind of description to go unchallenged.

“We should always be judged by our achievements, not by our gender.”

Williams said it was a concept she had struggled with ever since she was a young girl dreaming of becoming the best tennis player in the world, not the “best ‘female’ tennis player” in the world.

The grand slam champion said the discussion of equal pay also frustrated her because she and other women have “done the same work and made the same sacrifices as our male counterparts”.

“I would never want my daughter to be paid less than my son for the same work. Nor would you,” she wrote.

A photo posted by Serena Williams (@serenawilliams) on Aug 21, 2016 at 7:53am PDT

Williams concluded by calling for women to actively pursue their goals, saying she hoped her story inspired others to “push for greatness” and show resilience.

“We must continue to dream big, and in doing so, we empower the next generation of women to be just as bold in their pursuits.”

You can read the full letter at The Guardian.