Obama spoke to a group of local youngsters Friday as part of her campaign against childhood obesity.
"I'm not really good or anything like that. That's the beauty of tennis," she said. "You don't have to be good to enjoy it. I love the game, and my skills are very questionable."
Obama then joined the kids in a tennis video game, table tennis and other activities at an indoor facility at Flushing Meadows designed to get young players excited about the sport.
Obama and a pint-sized partner played doubles against seven-time Grand Slam winner John McEnroe, who gave her some volley pointers. Williams, a 13-time major champ who advanced to the Open semifinals Thursday, rolled in after about 45 minutes and took over as Obama's doubles opponent.
"I've been trying to get to the U.S. Open my entire life," Obama said.
Joined by former and current players Billie Jean King, James Blake and Katrina Adams, Obama recalled that she didn't get into tennis until after law school because there were few courts where she grew up on the South Side of Chicago. Obama praised the U.S. Tennis Association's efforts to build kid-sized courts around the country and recruit more youngsters to the game.
Now she hopes she and her daughters will still be playing when they're in their 90s.
"It's a sport you can do forever," Obama said.
Obama later took in the Andy Murray-John Isner quarter-final match at Arthur Ashe Stadium.