* Frenchman ruled out of U.S. Open with knee injury
* Tsonga hurt his knee at Wimbledon in June (Adds quotes, background)
Aug 15 (Reuters) - Frenchman Jo-Wilfried Tsonga will not compete at this month's U.S. Open tennis grand slam after failing to recover in time from a knee injury.
The world number eight, who suffered the problem during Wimbledon in June when he was forced to retire from his second-round match against Latvia's Ernests Gulbis, said he needed to be able to play at 100 percent before returning to the tour.
"The thinking is simple, the doctors told me it would take two months for my tendon to be partially healed and five months for it to be completely healed," Tsonga, a quarter-finalist in New York in 2011, said on his website.
"Two months is the length of time (from the injury) until the U.S. Open. It would be stupid to go there knowing full well that I have no chance because I'm not prepared enough."
The 28-year-old, a crowd pleaser with his acrobatics around the court, said he needed to look after himself and was well aware of what sort of injury he was dealing with.
"I know exactly what it is I have. I know exactly how long it will take me until I am back on top. It is an injury that I have had before," he said.
The year's final grand slam takes place in New York from Aug. 26-Sept. 9. Tsonga said he was eyeing a return to competition in Metz on Sept. 16.