Former world number one John McEnroe believes Serena Williams is already the greatest female tennis player ever even if she fails to complete a calendar year Grand Slam at Flushing Meadows this week.
Williams, 33, is currently just two matches away from becoming the first player to manage that feat since Steffi Graf in 1988 and takes on Roberta Vinci in Friday's US Open semi-final.
"She's the greatest female player I've ever seen. She's proven it to me over and over," McEnroe told BBC Sport.
Williams, the world number one, already holds all four grand slam titles having won last year's US Open and a victory this week will also draw her level with Graf's tally of 22 grand slam titles, two short of Margaret Court's all-time number of 24.
"I think Serena is one of the all-time greatest athletes, period. Man or woman," added McEnroe, a four-time US Open winner.
"You're looking at an incredible athlete and we've got to enjoy this last year or two that we have her around, because there's going to be a huge void when she's gone."
In anticipation for Williams possibly winning the title on Sunday, tickets for the women's final sold out quicker than the men's showpiece for the first time, helping to increase TV viewing figures.
"It was a boost for the US Open in general. Obviously for women's tennis it helps a great deal because without Serena, where would they be right now?" said McEnroe.
"I bet you 98 percent of people in the States couldn't tell you who the other three semi-finalists are."