Australian Open champion Serena Williams [Images] worked harder than expected but came away with a 6-3, 7-5 victory over unseeded Angelique Kerber of Germany [Images] in the first round of the US Open Monday.

The eighth-seeded Williams, an eight-times Grand Slam winner, needed 77 minutes to dispatch the 19-year-old Kerber.

"I didn't play well at all, at all," said 25-year-old Williams, who has been bothered by a sore left thumb and was playing in her first match since losing in the Wimbledon quarter-finals to Justine Henin [Images].

Williams, often left short of breath after being sent corner to corner by the German lefthander, served for the match at 5-4 in the second set but Kerber rose to the occasion, scoring a service break set up by a crosscourt passing shot.

That seemed to motivate the American, who broke right back and then served out the match as 67th-ranked Kerber netted a forehand on double matchpoint.

Williams was at a loss to explain her lacklustre performance, joking that perhaps she was uptight about extending her streak of never losing in the first round of a grand slam to 32 events.

"No, it was me," she said when asked what the German did to throw her off balance.

"I think she definitely played a tough game but I just think I made a lot of errors. I didn't feel I moved my feet well. I felt like I was just flat footed. I felt like I was just heavy. I don't know."

Williams had 38 winners to just 12 for the German, but she also committed more unforced errors -- 26 to 12 for Kerber.

Next up for Williams is unseeded Maria Elena Camerin of Italy [Images], a 6-3, 7-6 winner against Tatiana Poutchek of Belarus.

"I'll be fine," Williams added.

• Complete tennis coverage
• All you want to know about Sania Mirza