New York: Serena Williams survived a rare bad day on the courts as she beat Spain's Maria Jose Martinez Sanchez to secure a spot in the third round of the U.S. Open on Thursday.

The Wimbledon and Olympic champion, an overwhelming favourite for the title, needed 82 minutes to dig out a 6-2, 6-4 victory over 30-year-old Martinez Sanchez.

Williams started superbly, winning 12 straight points as she led 4-1 but struggled thereafter, double-faulting six times and making a slew of simple mistakes.

Martinez Sanchez could not capitalise, squandering a 2-0 lead in the second set as Williams battled through to face Ekaterina Makerova of Russia.

"It was one of those days," she said. "I wasn't really happy with the way I was playing. I just wasn't happy out there today in general. I think I woke up on the wrong side of the bed."

Williams hurt her ankle on Wednesday while playing doubles with her sister Venus but it appeared to have little effect on her play against Martinez Sanchez.

She first hurt the ankle at the Australian Open in January.

"I was running for a shot and rolled it," said the 30-year-old American, seeking her fourth U.S. Open crown. "Story of my ankle life. But it was fine. I did a lot of ice and (elastic) tape, compression to keep the swelling out.

"I rolled it in Australia so I was a little nervous but it's fine."

Williams blasted 32 winners but had an uncharacteristic 24 unforced errors in the one hour, 22-minute match on Arthur Ashe Stadium.

Both Williams and Martinez Sanchez had seven break-point opportunities but while Williams was successful on four, the Spaniard could convert only on one.


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