They are saying that she hurt her back during the 2nd set.
3.44pm AEDT - SERENA WILLIAMS OUT OF OPEN
American 19-year-old Sloane Stephens has recorded the biggest win of her short career, ending world No.3 Serena Williams Australian Open titles hopes in a dramatic quarter-final.
Stephens recovered after dropping the first set 6-3 but hit back as Williams struggled with a back injury, taking the next two sets 7-5 6-4.
In an enthralling final set, Williams' condition slowly improved but Stephens went with her, matching the world No.3 both in the power stakes and also game by game on the scoreboard.
Stephens faced four break points at 3-3 but and scrambled to save three of them, but went to the well one too many times, netting an off-forehand to give Williams a service break and a 4-3 lead.
But the teen would not be silenced, breaking back immediately before holding serve despite Williams pressing her again at 4-4.
Williams missed what should have been a simple passing shot at 15-15 as she served to stay in the match, and when Williams dumped a regulation backhand into the net Stephens had two match points.
She needed only one.
Williams netted yet another backhand halfway up the net to mark the biggest moment in the 29th seed's career.
An overwhelmed Stephens said she couldn't believe she had taken out the woman whose poster had adorned her bedroom walls.
"I think I'll have a poster of myself now."
Serena Williams has just taken out her injury frustrations on one of her racquets, burying it into the court after losing a game to trail Sloane Stephens 2-1 in the third set of their quarter-final.
Williams destroyed the racquet and was issued with a code violation for her troubles.
2.52pm AEDT - INJURED SERENA DROPS SECOND SET
A wounded Serena Williams is in big danger of crashing out of the Australian Open after suffering a back injury and losing the second set to fellow American Sloane Stephens.
Williams appeared to injure her lower back while serving at 3-4, losing the game and leaving the court before returning, only for a nervous Stephens to hang on to take the 7-5 after Williams won the first 6-3.
Williams is unable to serve with anything like her usual power, although she appears to be moving OK early in the third set.
READ HOW THE INJURY UNFOLDED BELOW.
2.35pm AEDT - WILLIAMS INJURES BACK IN SECOND SET
Serena Williams appears to have injured her back and has left the court for treatment in the second set against Sloane Stephens.
Williams was 3-4 down in the set after taking the first 6-3 when Stephens framed a ball that effectively became a dropshot, forcing the world No.3 to charge at the net to hit an awkward backhand winner but simultaneously put the brakes on.
The move prompted an immediate cry of pain from Williams, who was unable to serve with any power for the remainder of the game, which she then lost to trail 3-5.
Stephens served for the set but a series of Williams winners grabbed the break back, but Williams immediately called for the trainer, who took her from the court for treatment.