World No.1 Ash Barty has suffered her second upset loss in seven days, being turfed out of the Adelaide International at her first appearance.

Seven days after losing an Australian Open quarter-final, Barty has been beaten by American Danielle Collins in the WTA tournament at Adelaide’s Memorial Drive.

Collins prevailed 6-3, 6-4 on Wednesday night as the Australian top seed’s title defence fell at the first hurdle.

Barty took a wildcard entry to defend her Adelaide title after her Australian Open loss last Wednesday to Karolina Muchova.

But the Queenslander, after being gifted a first-round bye, fell in a hole after promising starts to both sets against Collins.

In the first set, Barty skipped to a 3-1 advantage but lost the following five games.

And the second followed a near-identical theme: Barty led 4-1 before Collins took the next five games.

The defending champion’s loss came as a series of seeds fell on Wednesday to underdogs, including Australia’s Storm Sanders.

The world No. 292 Sanders ousted seventh-seed and world No.28 Yulia Putintseva, winning 6-4, 5-7, 6-1.

Putintseva joined sixth-seed Petra Martic and eighth-seed Wang Qiang in making second-round exits.

American 16-year-old Coco Gauff stunned Martic 5-7, 6-3, 6-4 while Austrian Jil Teichmann downed Qiang 3-6, 6-3, 6-3.

And fourth-seed Elise Mertens didn’t even make it on court – the Belgian world No.17 withdrew from the tournament because of a right shoulder injury.

The unheralded Sanders advances to a quarter-final against second-seed Belinda Bencic who, after a first-round bye, breezed past Japan’s Misaki Doi to triumph 6-1, 6-3.

The Swiss 23-year-old, a beaten quarter-finalist in Adelaide last year, took six consecutive games from early in the first set to take a stranglehold on the match.

“I really like this court, it’s only my second time here in Adelaide but I love this tournament, I feel very comfortable here,” Bencic said in an on-court interview.