Agnieszka Radwanska has hailed the performance of Bethanie Mattek-Sands after her surprise defeat at the Sydney International.

The defending champion suffered a shock upset in round two on Tuesday as she lost 7-5 6-2 in straight sets to the American qualifier.

"She was really playing aggressive," Radwanska, who was suffering from a slight shoulder injury, told reporters. "Didn't matter if it was my first serve, second serve, or anything, she was really going 100% for every ball.

"It's not like I'm playing bad. I'm not worried about that. I just think not my day. Sometimes you have to lose. I think every week is different story. You start over and over again.

"Every tournament isn't the same thing. You're not winning every week every match. Normally you just to have to start again, not really think what was happening in the last week."

Mattek-Sands will now face fellow American Madison Keys in the quarter-finals.