Simona Halep suffers shock defeat as first seed out of Australian Open

Halep, ranked fourth at Melbourne Park, became the first seed to be bundled out of the season-opening grand slam, falling to the power-hitting American in 75 minutes.

She sought medical advice after the first set and was seen flexing her left knee throughout the second.

"I had pain at my knee," Halep told reporters. "For me, in the second set, was difficult to move anymore, but she deserved to win. She was aggressive, and she hit very strong."

Halep said she had been battling the problem since the WTA Tour Finals in Singapore. She had no issues during the off-season but it flared up again in Shenzen two weeks ago and she had been trying to control the pain since.

"I had some anti-inflammatory (medication) before the match and the previous days," she said. "I can play about 45, 50 minutes without pain. And then it comes.

"Today it was about 5-3 in the first set, so then it was tough to fight ... and I couldn't do what I wanted.

"I didn't see the doctor yet. I need an MRI. Probably need some time off to get it well, recovered, because it's difficult to play with the pain and the knees are dangerous," she added.

The injury compounded her frustrations at Melbourne Park.

While she reached the quarter-finals in 2014 and 2015, Monday's defeat was her fourth first-round exit in seven main draw appearances. She was beaten by Chinese qualifier Zhang Shuai last year.

Rogers, who beat former top-10 player Eugenie Bouchard two weeks ago in Brisbane, was overjoyed at her victory and believed making the quarter-finals at Roland Garros last year had helped with her confidence.

"The biggest thing I took away from that was just that I can compete with the top players in the world and I'm good enough," the 24-year-old said.

"I think I played great today, trying to be aggressive going out there. My game plan worked. Just move forward, hit your shots, be really aggressive.

"When I'm doing that, I am playing well."