Serena Williams said her "innate" fighting spirit got her through a huge battle with Simona Halep in a thrilling fourth-round clash at the Australian Open.

Williams has dominated Halep over the years and looked set for a crushing victory, but the world number one pushed her all the way on Rod Laver Arena.

The 23-time grand slam champion came through a major test to reach her 50th major quarterfinal, winning a gripping contest 6-1 4-6 6-4 on Monday.

"It was an intense match and there were some incredible points. But I love playing tennis. It's really cool to be back out playing on this court. I really needed to elevate my game," said the 37-year-old.

"She's the number one player in the world. I'm such a fighter. I just never give up. It's definitely something that's innate.

"It's a miracle I'm here and I get to do something I enjoy. This is my job and it's a super pleasure. That keeps me motivated and fighting for every point."

Seven-time Australian Open champion Williams will face Karolina Pliskova in the last eight.