Top seed Serena Williams powered into the semi-finals this morning - but was denied a showdown with elder sister Venus in Melbourne.

Serena had to save set points in her second-round match and lost the opening set in rounds three and four, but made no such mistake against 2014 finalist Dominika Cibulkova on Rod Laver Arena.

The 18-time grand slam singles winner took the opening set with two breaks of serve and fired down a total of 15 aces to complete a 6-2, 6-2 victory in 65 minutes. Venus had earlier lost in three sets to Madison Keys, a player 15 years her junior.

"I knew I had to be good today because she was in the final and plays really well on this court and I'm just happy to be through," Serena said in an on-court interview which had to be cut short due to a cough she has been struggling with for the last few days.

Williams later said in her post-match conference she could be suffering from the virus which has seen fellow-American Varvara Lepchenko hospitalised twice in the past few days.

"I've felt better, but I'm feeling okay," Serena said. "I'm just happy to still be in the tournament. "I've been sick the past few days. It's just getting worse and worse. But hopefully it will start getting better.

"I heard it's a virus going around with a lot of the players. I think I caught it."

Cibulkova admitted she was comfortably second best on the day, adding: "It was a tough match for me today. She was just playing really well today, I have to say.

"She was putting so much pressure from the serve and return. I didn't have a chance to play my game and felt under so much pressure. It was a good day for her."

Serena, 33, will take on fellow-American Keys in the last four after the teenager overcame a major injury scare to qualify for her first grand slam semi-final.

Keys, who is coached by former world No.1 Lindsay Davenport, needed lengthy treatment off the court and went on to lose the second set to her idol Venus Williams in their quarter-final.

However, the 19-year-old regrouped and twice came from a break down in the decider to win 6-3, 4-6, 6-4 and prevent Venus from becoming the oldest semi-finalist at a grand slam since 39-year-old Billie Jean King at Wimbledon in 1983.

Keys had never previously gone beyond the third round and feared the thigh problem which forced her to retire at that stage of Wimbledon last year would again ruin her prospects in Melbourne.

The world No.35 had taken the opening set in 29 minutes, but fell 4-1 behind in the second and then left the court to receive treatment on an injury to her left adductor.

Keys returned to the court with her thigh heavily strapped, but proceeded to win the next three games before Williams broke back and then served out for the set to force a decider.

Seven-time grand slam singles winner Venus, 34, twice moved a break up, but Keys began to find winners from all over the court and won the last three games to win a rollercoaster match featuring 12 breaks of serve.

"It was definitely kind of a flashback to Wimbledon for me," Keys said. "I have had some problems with that part of my leg so it was kind of an overwhelming moment. It was kind of scary.

"But luckily I was able to catch it before I did any real damage to it. Luckily the pain-killers and adrenalin in the end kind of helped me get through it.

"I felt it since the beginning (of the match). It's been tight, but it's been something that 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 tore it, which ultimately made me have to withdraw. At that moment (today) it was kind of a panic of, I need to get some tape on this so I don't do that again."

Speaking about reaching the semi-final, Keys added: "It definitely feels amazing. It's one of those things where you want to feel this way all the time.

"But it's not this unbelievable excitement either because you want to keep winning and you want to keep doing better. I am very happy and I am very excited, but also not getting too far ahead of myself and being too content where I am."

Venus was disappointed to lose in her first grand slam quarter-final since Wimbledon 2010, but could reflect on an encouraging start to the year following victory in Auckland earlier this month.

"It's definitely been a great last four weeks," she 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."

A Russia-United States final is guaranteed with second seed Maria Sharapova taking on compatriot Ekaterina Makarova, the No.10 seed, in Thursday's first semi-final.