American teenager Madison Keys shrugged off a leg injury to advance to her maiden grand slam semi-final, outlasting countrywoman and 18th seed Venus Williams 6-3 4-6 6-4 on Wednesday.

Keys was struck down by what appeared to be a groin or thigh injury during the second set, which also required a medical timeout.

But the 19-year-old dug deep, tallying 34 winners to Williams' 10, to earn her first win against the seven-time grand slam champion after two hours on court.

Keys also becomes the first American teenager since Serena Williams at the 2001 US Open to reach a grand slam semi-final.

Williams settled quickest of the two players but it was Keys who broke the deadlock in the seventh game of the opening set.

Three forehand winners on Williams' serve saw the American teenager take a 4-3 lead.

Keys " well and truly in her groove with 14 winners to her opponent's four " then held serve before breaking Williams again to love for first-set honours after 29 minutes of play.

It was only the second time Williams had dropped the first set at the Australian Open this year.

But just as Williams had done against Camila Giorgi, she regained the ascendency in the second set.

Two double faults from Keys, who had just a 22 per cent success rate on her second serve, handed Williams the early break.

Keys " clearly rattled and out of sorts with 11 unforced errors " dropped serve again as Williams claimed the double break for a commanding 4-1 lead.

The former then called for the trainer before taking a medical timeout with what appeared to be an injury to her upper left leg.

That brief moment off the court gave Keys the chance to regroup and it paid instant dividends as she broke back to love following four consecutive unforced errors from Williams.

But she was pegged back by Williams, who secured the break at the third time of asking in the ninth game before serving out the set and levelling the match with an ace.

Williams and Keys exchanged breaks at the start of the deciding set to be level at 3-3.

The pair continued to go toe-to-toe with each other, Keys broken in the seventh game, only to break back immediately.

And Keys managed to break again, this time it proved decisive as she won the match to love on Williams' serve.