Serena Williams of the US reacts after defeating Dominika Cibulkova of Slovakia to win their women's singles quarter-final match at the Australian Open 2015 tennis tournament in Melbourne January 28, 2015. [Photo/Agencies]

MELBOURNE - Madison Keys got one Williams sister out of the way on Wednesday at the Australian Open. On Thursday, the 19-year-old American will try to make it two, and this sibling might be a little tougher, even with a cold.

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 past 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.

"I've been sick the past few days, and it's just getting worse and worse, but hopefully it will get better," Williams added later. "I heard there's a virus going around with a lot of the players, and I think I caught it."

There had been some hope that the semis 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.

The semifinalists from the other half of the women's draw have already been determined - No. 2 Maria Sharapova will play No. 10-seeded Ekaterina Makarova, also on Thursday.

Serena's win doesn't bode will for the other three women left - all five past times Williams has advanced to the semifinals at Melbourne Park, she's won the tournament.