Serena Williams was just days removed from the emotional rollercoaster of a US Open finals loss to her sister Venus when tennis and all its trappings suddenly became irrelevant.

Serena Williams

Serena Williams had departed New York and was in Washington when the September 11, 2001 attacks brought down Manhattan's Twin Towers.  "I think 'fear' is a good word to describe how I felt," said Serena Williams. "I was really scared."

Ten years on, Serena is back at the US Open tennis championships and has set up a semi-final clash against  World No 1 Caroline Wozniacki.

Denmark's Wozniacki, whose runner-up finish at Flushing Meadows in 2009 remains her best Grand Slam finish, beat German Andrea Petkovic 6-1, 7-6 (7/5). Serena Williams wrapped up a 7-5, 6-1 victory over 20-year-old Russian Anastasia Pavlyuchenkova.