Keys overcame a left thigh injury to beat No. 18-ranked Venus Williams, 6-3, 4-6, 6-4, and advance to the last four against top-ranked and 18-time major winner Serena, who had little trouble beating last year's finalist, Dominika Cibulkova, 6-2, 6-2.
Serena Williams made the win look easy despite suffering from a bad cold the last few days. She spoke with a hoarse voice in her post-match interview on court.
“I have to keep my answers short because I keep coughing,” Williams said.
There had been some hope the semifinals might feature an all-Williams matchup at a major for the first time in more than five years. The Williams sisters last met in a Grand Slam match at Wimbledon in 2009, when Serena won the final.
Serena's win doesn't bode well for the other three women left — all five past times Williams has advanced to the semifinals at Melbourne Park, she has won the tournament.
Keys received treatment on her leg after dropping serve in the second set to give Venus Williams a 4-1 lead. After the medical timeout, she came back to break Williams' serve twice to level the set at 4-all, but Williams then broke her next service game and served out the set, sending it to a decider.
Keys was also behind 3-1 in the final set before breaking Williams' serve three times in a row to close out the match.
The match featured 12 service breaks, seven won by Keys, and 83 combined unforced errors.
The loss ended an encouraging 10 days for Venus Williams at Melbourne Park. She hadn't advanced to the quarterfinals of a Grand Slam since the U.S. Open in 2010 but is 9-1 to start the new year after capturing her season-opening tournament in Auckland, New Zealand, before the Australian Open.
Keys was pleased to be through to the semifinals.
“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.”
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.”
On the men's side, defending Australian Open champion Stan Wawrinka beat Kei Nishikori, 6-3, 6-4, 7-6 (6), in their quarterfinal match. Top-ranked Novak Djokovic was set to play Milos Raonic in a night match.