MELBOURNE (REUTERS) - American teenager Sloane Stephens caused the biggest upset at this year's Australian Open when the 29th seed rallied to overcome an injury-hampered Serena Williams in the quarter-finals on Wednesday.

Stephens prevailed 3-6, 7-5, 6-4 to set up a semi-final against defending champion Victoria Azarenka, the 19-year-old maintaining her focus to see out the victory after the five-time champion jarred her back in the second set and left the court for medical treatment.

Williams, the pre-tournament favourite, had smashed a racket in frustration early in the deciding set and while her anger appeared to motivate the 31-year-old enough to break Stephens' serve, her opponent showed plenty of maturity and broke straight back.

Stephens then held and broke Williams in the final game to set up a last-four encounter with Azarenka, who won a first-set war of attrition before crushing Svetlana Kuznetsova 7-5, 6-1.

The world No. 1 was pushed to the wall by the 27-year-old Russian, who played more like a two-time Grand Slam winner than her current world No. 75 ranking.