Draw opens up for Serena after Sharapova stunned by Pennetta

Serena Williams was already a hot favourite to win the US Open and the odds on the American grew even shorter last night when Maria Sharapova was beaten in the third round by Italy's Flavia Pennetta. Sharapova, who lost 6-3, 3-6, 6-4, was Williams' most likely challenger for the title, though you would never have guessed it from this ragged display.

The former champion made 60 unforced errors, hit 12 double faults and was broken seven times. An erratic serve has been her biggest problem ever since she had shoulder surgery three years ago and at times there was a look of resignation on her face as it again contributed significantly to her defeat.

Having lost the first four games of the opening set, Sharapova played much better in the second, although she continued to struggle on her serve. When the 24-year-old Russian levelled at 4-4 in the final set, having trailed 4-1, it seemed that Pennetta might allow victory to slip from her grasp, but the 29-year-old Italian held serve for 5-4 and then broke to love in the final game, in which her opponent began with two double faults. "I didn't feel comfortable with most of my game today," Sharapova said afterwards. "I made way too many unforced errors."

She added: "It's disappointing to lose in the middle of nowhere. It's disappointing to lose in New York. Losing isn't fun for anyone because we work to win. We don't work to try to lose. So when we're faced with a position where we can win and we don't in the end, it's tough."

Pennetta, the world No 25, reached the quarter-finals here in 2008 and 2009. She now plays China's Peng Shuai, the No 13 seed, who beat Germany's Julia Görges 6-4, 7-6.

Vera Zvonareva, the second seed, reached the last 16 but struggled for the second match in succession. The Russian, who lost to Kim Clijsters in last year's final, took two hours to beat Spain's Anabel Medina Garrigues 6-4, 7-5, having been taken to three sets by Kateryna Bondarenko on Wednesday.

"I feel like I was playing some good tennis and then there were a few moments where I felt like I disappeared,' Zvonareva said. "There were a couple of times, a couple of downs throughout the match. That's something I'm not really happy about but I came up with some good tennis and good shots. That's something that I need to focus on, especially if I want to win my next match."