Serena Williams has suffered a back spasm during her Australian Open quarter-final with Sloane Stephens and is now in danger of crashing out of the tournament.

The world number three pulled up as she was a set and 4-3 down on her serve after chasing down a ball at the net.

The 15-time Grand Slam champion squealed in pain when she quickly applied the brakes to stop herself from ploughing into the net, which would have seen her concede the point.

Williams went off court for a three-minute medical timeout, but was suffering from impaired movement as she dropped the second set.

The decider is currently poised at 2-2.