Sara Errani and Roberta Vinci to be foes... for a day

Sara Errani, pictured, has won seven doubles titles this year with Roberta Vinci (Getty Images)

Sara Errani and Roberta Vinci, who are doubles partners and best friends, were planning, as usual, to go to dinner together here last night. Today, however, the two Italians will be on opposite sides of the net when they meet in the quarter-finals of the US Open. In the fourth round on Monday, Errani beat Angelique Kerber, the No 6 seed, while Vinci knocked out Agnieszka Radwanska, the No 2 seed and Wimbledon runner-up.

Errani and Vinci have won seven doubles titles together this year and in reaching the doubles semi-finals here yesterday they ensured that they will move to No 1 and No 2 respectively in next week's updated doubles world ranking list. They have also enjoyed singles success. Errani, 25, has also won four singles titles this year and reached the final of the French Open. Vinci, 29, has won only one singles title but had never gone this far in 31 previous appearances at Grand Slam tournaments. "She's my best friend, so I'm very happy to play against her," Vinci said. "I'll be mean and so will she. And then afterwards we'll be friends, just like before."

The other quarter-final in the Italians' half of the draw will see Serena Williams face Serbia's Ana Ivanovic.

Victoria Azarenka became the first woman to reach the semi-finals when she completed a hard-fought 6-1, 4-6, 7-6 victory over Sam Stosur. The win, on a day affected by rain delays, guaranteed that Azarenka will retain her world No 1 ranking at the end of the tournament. She next plays the winner of yesterday's later match between Maria Sharapova and Marion Bartoli.