Agnieszka Radwanska insists she can still contend for Australian Open honours despite carrying a worrying shoulder injury and making a meal of her title defence at the Sydney International.

American qualifier Bethanie Mattek-Sands, ranked 48th, upset Radwanska 7-5 6-3 on Tuesday, marking the first time since 1990 that the women’s defending champion has lost her opening match at the century-old tournament.

The world No.5 received treatment on her serving shoulder during last week’s Hopman Cup in Perth and, while loath to blame the injury for her defeat, admitted it was still bothering her.

“Maybe a little bit, but I have good painkillers,” Radwanska said.

The non-sanctioned Hopman Cup aside, Radwanska hasn’t won a set – let alone a match – since October after also losing all three of her round-robin encounters at the season-ending championships in Istanbul.

The former Wimbledon runner-up’s run of outs is a far cry from last year when the Pole arrived at Melbourne Park for the season’s first grand slam riding a nine-match winning streak after picking up back-to-back titles in Auckland and Sydney.

But the 24-year-old is refusing to panic, claiming three wins at the Hopman Cup exhibition event is proof enough she’s not playing badly.

“Every week is different story. You start over and over again,” Radwanska said.

“You’re not winning every week every match. I think just couple of guys can do it.”

Mattek-Sands’ surprise second-round win – after she beat higher-ranked Canadian Eugenie Bouchard and Radwanska enjoyed a first-round bye – thrust her into a quarter-final with fellow American Madison Keys, a 6-0 3-6 7-6 (7-4) victor over Croatian wildcard Ajla Tomljanovic.

Czech second seed Petra Kvitova and German fifth seed Angelique Kerber are also through to the quarter-finals.

Kvitova thrashed US qualifier Christine McHale 6-1 6-0 in one hour neat to book a likely last-eight date with Danish sixth seed Caroline Wozniacki, while Kerber downed big-hitting Estonian Kaia Kanepi 6-3 6-4 to set up a quarter-final with Spaniard Carla Suarez Navarro.

Wozniacki plays her second-round match later on Tuesday against Czech Lucie Safarova.