World number one Roger Federer's 12-match winning streak ended when he went down 6-3, 6-3 to 17-year-old Spaniard Rafael Nadal in the third round of the Nasdaq-100 Open on Sunday.

Before he ran into Nadal, the Swiss player had won 28 of his last 29 matches stretching back to the Masters Cup last November, including his Australian Open victory and back-to-back titles in Dubai and Indian Wells.

Former world number ones Lleyton Hewitt and Jennifer Capriati were also shown the exit door on a day of upsets.

Hewitt suffered a 6-4, 7-5 defeat by Romania's Andrei Pavel while women's fourth seed Capriati was stunned 6-2, 6-4 by Greece's Eleni Daniilidou.

Nadal and Tim Henman (who defeated Federer in Rotterdam in February) are the only men to have beaten the world number one in four months.

The Spanish teenager showed no fear, breaking serve early in the first set and hitting with relentless aggression until Federer's serve cracked again at 2-3 in the second. Nadal, seeded 32, then served the match out with an assurance which belied his years.

"He hit some great shots and he was the better player today. I couldn't quite play the way I wanted to," Federer told reporters.

"I'm very happy because I played one of the best matches of my life," said Nadal through a translator.

Second seed Venus Williams was let off the hook in her match with Daniela Hantuchova. The Slovak served for the first set three times only to lose it on a tiebreak.

American Williams made the most of the reprieve, romping through the second set for a 7-6 (7-0), 6-2 win.

Her sister Serena, top seed in the women's draw, also flirted with danger before overcoming Russian Elena Likhovtseva 6-1, 4-6, 6-3 to secure a place in the last 16.

Hewitt struggled to find his service rhythm throughout the match and his misery was compounded by his inability to cope with Pavel's powerful groundstrokes.

"He played well but I felt like I didn't quite take my opportunities when I had them," said the Australian 17th seed.

"I put myself into a position where I had to try to claw my way out and I wasn't quite able to do that... it's a little bit disappointing."

DOUBLE FAULTS

Hewitt served 10 double faults during the match, the most costly of which came on Pavel's fifth match-point.

After going a set and 4-2 down, Hewitt appeared poised to pull of one of his trademark comebacks when he broke back for 4-5. However, his serve faltered again as Pavel completed a memorable victory.

Capriati, who is competing in her third tournament of the season after missing the Australian Open with a back injury, failed to negotiate Daniilidou for the second time in a month.

The American was also knocked out by the Greek in Dubai last month.

Former world number one Serena was playing only her second match after being sidelined for eight months with knee problems and her error-strewn performance against the world number 42 smacked of rustiness.

"I didn't play very well," said the American. "I was giving a lot of stuff away out there today. I never felt like I was going to lose though. It was always like, 'when am I going to win?'

"I'm glad I had that experience because I have been a little short of match-play lately. There is nothing like match play to torque (condition) your body in different ways. With each match I'm getting better."

"I think Serena needs more matches but she fights to the end and you see how much she wants to win," said Likhovtseva.

"She is not in such great shape right now and I think if I had been playing my best tennis I would have won."

Serena, who underwent knee surgery in August, is playing her first tournament since winning Wimbledon last July.

She next plays 17th-seed Maria Sharapova for a place in the quarter-finals.

In the men's draw, third seed Guillermo Coria came through a gruelling all-Argentine battle with a 6-2, 3-6, 6-3 win over Juan-Ignacio Chela.