Venus Williams of the US waves to the crowd as she walks off the court after being defeated by compatriot Madison Keys in their women's singles quarter-final match at the Australian Open 2015 tennis tournament in Melbourne January 28, 2015. [Photo/Agencies]

"It's amazing, you just have to embrace the moment," said Keys, who is now coached by three-time major winner Lindsay Davenport. "And I get to enjoy another moment next round."

Keys said the injury was the same one that forced her to retire from her third-round match last year at Wimbledon.

"It was definitely one of those things where it wasn't nearly as bad as Wimbledon, but it was that nightmare of `I don't want this to happen again,"' she said. "Luckily the pain meds kicked in."

She later identified the injury as a left adductor, and was optimistic it wouldn't affect her in her match against Serena.

"I think it's one of those things where all of us have dealt with injuries before," she said. "It's one of those things where it's probably going to hurt. I'm probably going to have tape on it, but I'm just going to do my absolute best and enjoy the moment."

Serena Williams played one of her best matches of the tournament against Cibulkova, saving both break points she faced and hitting 58 winners.

She expects a tough match against Keys.

"I think she likes the surface," Williams said. "I'm just happy to be in the semis, and whatever happens an American will be in the final."