Serena Williams shrugs off John McEnroe claim she'd struggle against men

Serena Williams responded to tennis great John McEnroe's claim that she couldn't beat most men on the professional tour, writing on Twitter Monday that the commentator should "keep me out of your statements that are not factually based."

McEnroe told National Public Radio over the weekend that Williams is the best women's tennis player in history -- "no question." But when asked whether she was the sport's best ever, regardless of gender, he made clear he didn't think so.

He said he thought Williams could beat some men, "but if she had to just play the circuit -- the men's circuit -- that would be an entirely different story." He said if she tried to be part of the men's tour, "she'd be like [ranked No.] 700 in the world."

Williams has won 23 women's Grand Slam singles titles — a record for the Open era, which dates back to 1968. She is taking a break from the tennis tour because she is pregnant and plans to return to competition in 2018.

McEnroe won seven men's Grand Slam singles titles, all between 1979 and 1984. He interview on NPR was part of a promotional tour for his new memoir, "But Seriously."

The Associated Press contributed to this report.