By Martyn Herman

LONDON (Reuters) - The United States will have six players in the men's and women's singles fourth round on Independence Day on Monday -- the nation's best showing at Wimbledon for 12 years.

Sam Querrey, whose stunning defeat of defending champion Novak Djokovic reverberated around the world, will be joined by pal Steve Johnson in the men's last 16.

Champion and world number one Serena Williams leads the charge in the women's draw, where she will be joined by older sister Venus, Coco Vandeweghe and Madison Keys.

Sloane Stephens and John Isner narrowly missed out on joining them after losing marathon third-round matches on Sunday but the mood in the American camp is buoyant.

"Well, it's not dead, and I'm really happy that we're having positive conversations about it for once," ninth seed Keys, who faces Simona Halep on Monday, told reporters.

Things reached a low point in 2014 when not a single American reached the fourth round of the singles.

While Serena, who plays Stephens's conqueror Svetlana Kuznetsova on Monday, has ensured a procession of grand slam silverware over the past decade, no American man has won a major since Andy Roddick landed the U.S. Open in 2003.

That was the same year Andre Agassi won the last of his eight grand slam titles in Australia.

Querrey, who plays unseeded Nicolas Mahut, and Johnson, who takes on Roger Federer, are long odds to end that drought here.

But with 11 men starting in the main draw, including rising teenager Taylor Fritz and vastly improved world number 25 Jack Sock, there is clearly strength in depth.

"I mean, we're all right there, you know," Johnson, who has reached the fourth round of a slam for the first time, said.

"I think we get criticized a bit for not winning slams, which is fair, because back in the day with Connors and McEnroe and Sampras and Agassi and those guys.

"It's one of those things. It comes in waves, and hopefully we have a lot of good guys coming up and hopefully they can push us and we can help them any way we can."

Vandeweghe said the so-called lack of rising Americans has been overblown.

"I can only speak for the women's side. For the last two, three, four years, there's been a few players kind of knocking on the door of top 20," she said.

"Madison just made top 10. I think the whole stigma of U.S. tennis is non-existent and is not relevant anymore."

The U.S. is not the only nation with a healthy representation. France has four men in the last 16, Mahut, Jo- Wilfried Tsonga, Lucas Pouille and Richard Gasquet.

Russia has Elena Vesnina, Ekaterina Makarova, Kuznetsova and Anastasia Pavlyuchenkova in the women's last 16.

Spain, however, has flopped, with Carla Suarez Navarro their only survivor in the either singles draw.

(EDiting by Ed Osmond)