Angelique Kerber's first WTA Tour outing since her brilliant Australian Open triumph ended in disappointment as the top seed suffered a shock 7-5 6-1 loss to Zheng Saisai at the Qatar Open.

China's Zheng Saisai returns a shot to Germany's Angelique Kerber during the first round at the Qatar Open on Wednesday.  [Photo/Agencies]

Kerber upset the odds to defeat Serena Williams in the Melbourne showpiece last month, but the German - who has played in the Fed Cup but not on Tour since her win - looked flat in Doha.

Zheng deserved credit for a tenacious showing and accurate ground strokes, while Kerber's 38 unforced errors gave the Chinese plenty of cheap points en route to the biggest win of her career.

Next up for Zheng is a tricky contest with wildcard Eugenie Bouchard.

The Canadian continued to show signs that she is back to her best by defeating Denisa Allertova 7-6 (7-0) 7-5.

There was also disappointment for Simona Halep, Belinda Bencic and Lucie Safarova on a day of upsets.

Second seed Halep was up a set and 4-1 in the second before losing 11 out of the next 12 games in a 6-7 (1-7) 6-4 6-1 to qualifier Elena Vesnina.

Bencic - the sixth seed - was put to the sword 6-4 6-2 by Coco Vandeweghe, a week after losing in the first round in Dubai to Jelena Jankovic – herself beaten 3-6 6-4 7-5 by Monica Niculescu.

As for defending champion and seventh seed Safarova, she lost 7-6 (8-6) 7-5 to wildcard Cagla Buyukakcay.

It was a straight-forward outing for third seed Agnieszka Radwanska as she saw off Kateryna Bondarenko 6-4 6-4, but ninth seed Roberta Vinci had to save three match points in a 2-6 6-4 7-6 (7-3) win over Daria Kasatkina.

Garbine Muguruza easily saw off Nao Hibino 6-2 6-0, a win that will see her move above Halep in the rankings, while Caroline Wozniacki's resurgence continued in a 6-3 6-3 victory over Daria Gavrilova.

Petra Kvitova and Carla Suarez Navarro, the fifth and eighth seeds, were comfortable winners. The former beat Czech compatriot Barbora Strycova 7-6 (7-2) 6-4, while the latter saw off Donna Vekic 7-6 (7-5) 6-4.

Timea Babos put paid to 16th seed Sara Errani's hopes in a 6-4 6-1 victory, Andrea Petkovic was a 6-3 6-3 winner against Margarita Gasparyan, Julia Ostapenko beat Svetlana Kuznetsova 7-6 (7-5) 6-0 and Timea Bacsinszky overcame Yulia Putintseva 6-2 7-6 (7-0).