Olympic champion Victoria Pendleton crashed in the first bout of her best-of-three sprint semi-final with old adversary Anna Meares of Australia at the Track Cycling World Championships in Melbourne.
Meares, the defending champion, rounded Pendleton entering the finishing straight.
The pair clashed elbows before Pendleton lost control and hit the track hard as the Australian went on to take a 1-0 lead.
The 31-year-old from Bedfordshire was attended to quickly and appeared unharmed as she cooled down on the stationary rollers in preparation for the second race.
Pendleton appeared to have been beaten comfortably in the second bout with Meares, but was granted a reprieve.
Meares seemed to veer off her inside line and Pendleton was impeded before the Australian accelerated away.
But the result was overturned for Meares going outside the sprinters' lane.
It meant there would be a third race between the pair to decide who would advance to the final.
In the women's scratch race, a non-Olympic 10-kilometre (40 laps) event, Dani King finished fourth.
The 21-year-old, who on day two was part of the British squad which won team pursuit gold in a world record, was in the mix heading towards the line, but Poland's Katarzyna Pawlowska was strongest to triumph.Australia's Melissa Hoskins was second, with Kelly Druyts of Belgium third as King was squeezed out of the medals.