Published Monday, 20 August 2012
The five-piece reunited to sing Wannabe and Spice Up Your Life at the show on 12 August, but Halliwell admits her mind was on her late dad, Laurence.
She tells Hello! magazine: "Sadly he died six months before the Spice Girls, when I was 21. So doing the Olympics on his birthday was so sweet and poignant for me.
"It was a wake-up call, dealing with that pain and loss, then turning it into drive."
Halliwell turned 40 on 6 August, and spent her special day in secret rehearsals for the show with her bandmates - and she couldn't have imagined a better celebration.
She adds: "My actual birthday was the most bizarre ever. It was the day we started rehearsing, so it was spent whizzing around on a taxi in an industrial car park wearing Wellington boots. The girls were very sweet and made me feel special. There was a cake, lovely presents - an elegant, pretty necklace - and singing of course.
"Doing the Olympics was my birthday present - the best gift ever. I just can't express how wonderful it was. It blew me away, it was absolutely a dream come true to kick off the next 40 years."