United States Tennis Association (USTA) president Katrina Adams labelled 21-time grand slam champion Serena Williams as "the greatest athlete of all time".

The 33-year-old Williams is bidding for a calendar Grand Slam when the US Open starts at Flushing Meadows on Monday.

Adams, the first African-American and former professional player to lead the USTA, lauded Williams for her achievements and ranked the world number one as the greatest ever.

"Serena Williams is the ultimate champion. She knows what it takes to get to the top," Adams told Omnisport.

"She started at the bottom from Compton, worked her way up through the ranks of the professional tour to number one in the world. She has been number one in the world for the last four years. She's been number one multiple occasions.

"But when you look at what she has done and where she is today, going for her calendar Grand Slam, that's an amazing story in itself but it also contributes to just the power that she has in her strength and the confidence that she has and the devotion that she has to sport."

Adams added: "Serena Williams is the greatest tennis player of all time, the greatest female athlete of all time, the greatest athlete of all time."

Williams has lost just two matches in an incredible 2015, while three of her opponents benefited from walkovers.

Adams said if any player was able to deal with the pressure of achieving a Grand Slam, it was Williams.

"What an amazing opportunity it would be for Serena to win the calendar Grand Slam, she put herself in the position to do that," she said.

"She's the first to do it since Steffi Graff in 1988. For Serena you can see just how rare this feat would be and with her winning last week in Cincinnati I think it's building her confidence.

"She only lost one match this year or maybe two and so coming into the US Open there is a lot of pressure on her but if anyone can do it, it's Serena."