Australian Open 2015: Serena Williams made the win look easy despite suffering from a bad cold. (Source: AP)

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.

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 broke Williams’ serve twice to level the set at 4-all, but Williams then captured the next two games to send 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 and had struggled to recapture her earlier form after being diagnosed in 2011 with Sjogren’s syndrome, an auto-immune disease that causes fatigue and joint pain.

But Williams is 9-1 to start the new year after capturing her season-opening tournament in Auckland, New Zealand, before the Australian Open.

“It already feels like a long season already, so many matches in a row,” she said. “But it’s a great start. Hopefully I’ll be able to keep this level up.”

Regardless of the quality of the error-strewn match, Keys was pleased to be through to the semis.

“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.”