Simona Halep fell victim to the first shock of the Australian Open after the number four seed was dumped out by American Shelby Rogers.

Halep had been one of the favourites to challenge for the title in Melbourne but the Romanian was beaten 6-3, 6-1 by Rogers in the tournament's curtain raiser on Rod Laver Arena.

It is the second year running Halep has endured a first-round defeat here after she lost to Japan's Shuai Zhang 12 months ago. Rogers goes through to meet either Australian wildcard Ashleigh Barty or Germany's Annika Beck in round two.

An analysis conducted by Tennis Australia's data team had Halep as the fastest female player on the circuit but she was powerless to resist Rogers, who hit 26 winners and managed four breaks of serve.

After the match, Halep admitted she was hindered by a bout of tendinitis, which had affected her left knee since the WTA Finals in October.

"Definitely she played well. I think she played a very high standard. I had pain at my knee," Halep said.

"I tried. I had some anti-inflammatory before the match, and the previous days. But when you have the tension of the match, official match, the pressure, it becomes harder.

"So I had harder pain and I couldn't do what I wanted."

Rogers, ranked 52nd in the world, has only once before played in the Australian Open main draw and on her last meeting with Halep, at the 2015 US Open, she lost in straight sets.

American Shelby Rogers celebrates her victory over Simona Halep (Getty)

"I think I played great, trying to be aggressive," Rogers said. "My game plan worked. Just move forward, hit your shots, be really aggressive. When I'm doing that, I am playing well."

Garbine Muguruza and Venus Williams avoided similar upsets but both were made to work hard as Muguruza edged past Marina Erakovic 7-5 6-4 while Williams beat Kateryna Kozlova 7-6 (7/5) 7-5.