UPDATE 1-Tennis-Serena retires injured, Tsonga through in Madrid

* Serena knee injury forces retirement against Schiavone

* Tsonga ousts Safin, Ferrer sees off Canas

* Dementieva through against Dominguez Lino (Adds Dementieva result)

By Iain Rogers

MADRID, May 11 (Reuters) - World number two Serena Williams retired from her first-round match against Francesca Schiavone at the Madrid Open on Monday with a knee injury after losing the first set to the Italian.

The American former number one said she did not want to harm her chances at the French Open starting later this month and added the knee problem was linked to a thigh injury that has been bothering her in recent weeks.

In the men's draw, ninth seeded Frenchman Jo-Wilfried Tsonga battled past his former idol Marat Safin 6-4 7-5 to set up a second-round clash with Croatian wild card Ivan Ljubicic.

Tsonga, 24, said he remembered watching the 29-year-old Safin, a former world number one, playing at the French Open and the Paris Masters, where the Russian won three times and finished runner-up once.

"For me he was like an idol before so it was really special to play against him and to win against him," Tsonga told a news conference.