PARIS (Reuters) - American teen-ager Andy Roddick was forced to retire from his much-anticipated French Open third-round clash with Australian sixth seed Lleyton Hewitt on Friday because of injury.

The match was poised at a set apiece and 2-2 in the third when 18-year-old Roddick, previously unbeaten on clay, turned his right ankle and injured his thigh as he fell when chasing a wide ball.

Ignoring pleas from coach Tarik Benhabiles and U.S. Davis Cup captain Patrick McEnroe to retire, Roddick got up to play on but after four more points he was forced to give up.

``I pulled it (the thigh) in the last match and I pulled it again,'' he told reporters as he left the court in tears to warm applause from the crowd.

Roddick overcame compatriot Michael Chang in a five-set marathon in the second round Wednesday, during which he fought off severe cramps in his hands, calves and groin.

Hewitt, who faces Argentine Guillermo Canas in the last 16, said he was disappointed for the crowd.

``We played two and half tough sets out there. No-one likes to win like this but it's nice to be through,'' said the Australian, who has now matched his best ever performance at Roland Garros by reaching the last 16.

``It's disappointing. It was pretty good tennis out there today. It was two very young players on the tour going at it head to head in a Grand Slam, that's what everyone loves.''

CANNONBALL SERVE

Roddick, who was thrashed in straight sets by Hewitt in the Miami quarterfinals in March, drew gasps from the crowd early on as he lashed his cannonball serves and crunching forehands all over the court.

The American, playing only his second Grand Slam tournament, lacked consistency, however.

Hewitt, playing the cannier game, was left kicking himself as he squandered seven break points -- six of them set points -- before the first set went into a tiebreak.

The Australian then wasted his seventh set point and Roddick, on his first, produced a thundering serve to take it 7-6 (8-6), pumping his fists in delight.

Roddick continued to misfire in the second set and finally Hewitt, on his 11th break point of the match, went 3-2 up, lost serve immediately but broke again for 5-4 before producing two aces to win the set and square the match.

TURNED ANKLE

It looked like another late-night marathon was beckoning until, at 1-2, Roddick vainly chased a game-winning shot from Hewitt, turned his right ankle over on the clay and collapsed in a heap clutching his right hamstring.

After a three-minute timeout for some strapping, Roddick, his shirt and baseball cap still covered in clay, bravely tried to continue his next service game but gave up at 15-40 and hobbled off.

It was a sad end to his run of 12 wins on clay which took in successive tournament victories at Atlanta and Houston and his first and second-round wins at Roland Garros, and he now faces a race against time to be fit for Wimbledon .