Two-times defending champion Serena Williams, in her black and white leopard dress, started quietly before roaring past 11th seed Flavia Pennetta 6-3 6-2 to reach the semi-finals of the US Open.

The world number one lost her first two service games to Pennetta to fall behind 3-0 before snapping to attention and rattling off the next six games to claim the first set.

“She was playing really well and she was playing very aggressive,” said Williams.

“So I had to just step up my level of game as well.”

Determined to make up for a disappointing grand slam season in which she failed to reach the quarter-finals in any of the three previous majors, Williams broke her fellow 32-year-old in the fifth and seventh games to advance in 63 minutes.

The top seed will meet Russian 17th seed Ekaterina Makarova, who reached her first grand slam semi-final with a 6-4 6-2 victory over 16th seed Victoria Azarenka, runner-up to Williams in the last two Opens.

Pennetta was disappointed to have lost, but not disappointed with the quality of her tennis against the 17-times grand slam singles winner.

“She was really good today. I think we played really good tennis tonight. I mean, really strong tennis in everything,” said the Italian.

“Today, I pushed so much and she came in with the best tennis, I think.”