John Pye

MELBOURNE, Australia: Serena Williams has never lost in the semifinals of the Australian Open. Neither has Novak Djokovic.

A key difference between the defending champions is that Williams beat the most-experienced and highest-ranked challenger in her half of the draw when she extended her streak to 18 consecutive wins against Maria Sharapova with a 6-4, 6-1 quarterfinal victory on Tuesday.

Williams said she wasn’t aware that she had won the Australian Open all six previous times she won her quarterfinal match, “but that’s good.”

“It’s not a stat that’s set in stone,” Williams said. “I still have to win two matches.”

Five-time champion Djokovic, after beating No. 7 Kei Nishikori 6-3, 6-2, 6-4, still has a daunting semifinal match against Roger Federer, who has won four of his 17 Grand Slam titles in Australia.

Federer reached his 12th Australian Open semifinal, and 39th in a Grand Slam event, with a 7-6 (4), 6-2, 6-4 win over No. 6 Tomas Berdych. He hasn’t gone past the semifinals at Melbourne Park since winning the title in 2010, but he’s a serious obstacle for Djokovic.

They’re 22-all in career head-to-heads, with Djokovic catching up since usurping Federer in the rankings.

The tiebreaker will be tricky. Djokovic lost only one of his 28 Grand Slam matches in 2015 — to Stan Wawrinka in the French Open final — and has won 37 of his past 38 matches at Melbourne Park, a run that includes four titles.

He beat Federer in the Wimbledon and U.S. Open finals last year, but Federer was back contending for titles.

“Any round feels like finals because of the fact that we are, you know, big rivals, we played so many times against each other,” Djokovic said. “There’s a lot of tension. There’s a lot at stake. I’m expecting a great fight in two days.”

In the women’s semifinals, Williams will be facing fourth-seeded Agnieszka Radwanska, who beat No. 10 Carla Suarez Navarro 6-1, 6-3. Radwanska has won one of her previous four semifinals at a major, but never won a Grand Slam title.

Williams is 8-0 against Radwanska, including their meeting in the 2012 Wimbledon final.

Williams said she couldn’t explain her 11-year domination of Sharapova — she has won 19 of their 21 meetings overall — except to say she rises to the big challenges.

“When I play her, I know automatically I have to step up my game,” said Williams, a 21-time major winner. “I think that makes me play better.

“When I’m forced to play better, I do well.”

Williams attacked Sharapova’s strength, targeting the five-time Grand Slam winner’s improving serve.

Sharapova had a career-high 21 aces in her previous win against No. 12 Belinda Bencic. Against Williams, she had three, and seven double-faults. Williams had 13 aces, hit 31 winners to 11, and broke Sharapova’s serve four times.

“She played quite explosive,” Sharapova said. “She was really explosive off the return. Yeah.”