Melbourne, Jan 26 (IANS): Top seed Serena Williams withstood a tense first set and a late match charge to take out fifth seed Maria Sharapova 6-4, 6-1 to enter the Australian Open tennis semi-finals here on Tuesday.

It was the American's 18th straight win against the Russian.

Sharapova came out firing to start, taking an early 2-0 lead on Australia Day. But Williams worked her way into the match in style, saving break points at 4-4 and winning seven games in a row at one stage.

"It was super intense," the 21 Grand Slam winner reflected later.

"She's (Sharapova) an incredibly intense, focused player who was No.1 and won so many Grand Slams for a reason.

"When you're playing someone who's so great, you have to come out with a lot of fire and intensity," she added.

Serena had to summon a doctor on court after the first set, but was nonetheless ruthless in the second.

She went close to a 6-0 win before Sharapova pulled back, even earning two break points for 5-2 as the top seed served for it.

"I've been playing this whole week aggressively, but I didn't start out playing that way today," the 34-year-old said.

"I just knew after the first set that I wanted to start playing the way I have been, that got me to the quarterfinals, so I was just trying to do that."

The World No.1 will face No.4 seed Agnieszka Radwanska in the semis.

"She's a great defender and a great girl. I'm going to do my best, and I have nothing to lose," she said. She then addressed the crowd: "Thank you guys for coming out; I hear you all, and it means a lot to me!"

Looking to tie Steffi Graf's record of 22 Grand Slam titles, Williams is also after a seventh Australian Open title.

"I'm here all the time; I have so many friends here. This is one of the few stadiums where I feel so welcome."