World number one Serena Williams has insisted that Sabine Lisicki is "not a pushover".

Williams went into their last-16 encounter at Wimbledon this afternoon as the favourite to advance, but her German opponent caused an upset by recording a 6-2 1-6 6-4 win.

However, despite the fact that she is ranked 23 in the latest standings, Williams has claimed that it is no reflection on the quality that Lisicki possesses.

"She's a great player, her ranking has no [bearing] on where she should be. She should be ranked higher," the American told reporters.

"She has a super game to play well on grass. I feel like I had an extremely tough draw today, of all the round 16 games I probably had the toughest one. Come on, guys, let's get with it, she's excellent. She's not a pushover."

Defending champion Williams had been aiming to win the title at SW19 for a sixth time.