Melbourne: Romanian tennis star Simona Halep admitted she is suffering from tendinitis in her left knee that contributed to her shock first round exit at the Australian Open on Monday.

Halep also said she will be sidelined for a lengthy period of time, reports Xinhua.

American Shelby Rogers' aggressive game plan made quick work of the fourth seed in straight sets 6-3, 6-1 in just 75 minutes in their opening round clash on Monday.

"I'm kind of used to that, playing the seeds and playing big matches right off the bat," World No.52 Rogers said after the match, adding she has always enjoyed playing on big courts with big crowds.

"I have always loved that atmosphere. It's a little bit different playing on such a big court and then having different pressures coming from everywhere. I think I have matured a little bit and gotten used to that."

Though bothered by the pain late in the first set, withdrawing from the match never entered Halep's mind as "it's not in my inside (sic)", the fourth-ranked player said.

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She also admitted Rogers "deserved to win" for her aggressive play and strong ball hitting.

"She was stronger today than me," Halep said.

The Romanian has been suffering from the injury since late last year, undergoing non-invasive treatment, while taking anti-inflammatory medication over the last couple of days, though she hasn't seen a doctor yet to determine if surgery is needed.

"I need an MRI (Magnetic Resonance Imaging)... (and) probably need some time off to get it well," Halep said.

"It's difficult to play with the pain, and the knees are dangerous, so I have to take care."

It's the second first round exit for Halep at the Australian Open after being downed by an emotional Zhang Shuai in straight sets 4-6, 3-6 last year.

Rogers will face home wild card Ashleigh Barty in the second round.