Madison Keys overcame a left leg problem to prevail against countrywoman Venus Williams on Wednesday but the American teenager said the injury is not new.

Keys was a 6-3 4-6 6-4 winner over Williams on Rod Laver Arena, though she was forced to do it the hard way after she was struck down by injury during the second set.

The 19-year-old required a medical timeout in the second set, while she had her left leg massaged in the following stanza.

Keys, however, battled through to set-up another all-American showdown, this time against world number one Serena Williams.

And Keys, who was still in pain afterwards with heavy strapping on her left leg, told reporters the injury was a reoccurrence of a problem that she carried into the grand slam at Melbourne Park.

"It was kind of scary. But luckily was able to catch it before I did any real damage to it," said Keys, who became the first American teenager to reach a grand slam semi-final since Serena Williams in 2001.

"Luckily the painkillers and adrenaline in the end kind of helped me get through it.

"I felt it since the beginning. It's been tight but it's been something that, you know, with some treatment it's been fine.

"Then one shot in the match, all of a sudden I felt it kind of really get tight. I thought I was close to pulling it.

"I ignored it at Wimbledon, and, you know, tore it, which ultimately made me have to withdraw.

"At that moment it was kind of a panic of, I need to get some tape on this so I don't do that again."

Wednesday's loss ended Venus Williams' stellar run in Melbourne and prevented a mouth-watering showdown against sister Serena.

While disappointed about the result, Venus " who suffers from the autoimmune disease Sjogren's Syndrome - was pleased with her Australian Open campaign, having reached the quarter-finals in Melbourne for the first time in five years.

"It's definitely been a great last like four weeks, I think," the 34-year-old veteran said.

"A lot of match wins, a lot of momentum, just a lot of great things happening that everybody wants to happen in their career.

"I think a lot of players would have been happy with this tournament. I'm happy with progress. I'm not happy with a loss, I'll tell you that."