UPDATE 1-Tennis-Open-Serena slams 'junior' display against Czech

* Serena made to sweat on cold day to reach second round

* Downs Zakopalova on her ninth match point

* I played horrendously, says the world number two

(adds quotes, details)

By Julien Pretot

PARIS, May 26 (Reuters) - Serena Williams rated her performance as "junior tennis" after she needed nine match points to end the bold challenge of Czech Klara Zakopalova 6-3 6-7 6-4 in the first round of the French Open on Tuesday.

"I played horrendous. I played junior tennis or even worse. I was a little nervous because I hadn't won a match on clay all year," Williams, who will next face Spain's Virginia Ruano Pascual, told reporters.

"I was just desperate for a win and it pretty much showed in my game."

At 3-0 down in the second set, Williams bombarded Zakopalova with a series of winners to move 5-3 up after pocketing 10 points in a row but it was not enough to crush the world number 100's morale.