Wozniacki, champion at Indian Wells in 2011, had overwhelmed Bencic 6-0 6-0 in their only previous meeting in Istanbul last year but it was a very different story at the California desert venue as the Dane struggled for consistency.

"Honestly, it was two completely different matches," Wozniacki told reporters after being broken twice in the opening set and once in the second at a sun-bathed Indian Wells Tennis Garden. "I went out there today and I didn't play well at all.

"She was steady. She took the ball early as normal, and she served pretty decently, but I didn't put three balls in play today. She only had to put balls back in my court."

Bencic, who celebrated her 18th birthday at Indian Wells last week, was delighted to clinch her first victory over a top-five player.

"In Istanbul I had maybe too much respect and I was afraid, nervous," said the Swiss, who gave notice of her potential by reaching last year's U.S. Open quarter-finals. "Today I really had a good game plan. I did what I had to do."

It was plain sailing, however, for sixth-seeded Canadian Eugenie Bouchard who used her potent forehand to great effect as she demolished American CoCo Vandeweghe 6-3 6-2.

The 21-year-old broke an error-prone Vandeweghe once in the opening set and twice in the second to complete a one-sided victory in just over an hour on the showpiece stadium court.

Bouchard, who reached the last four in Australia and France as well as the Wimbledon final in 2014, hit 13 winners against her hard-hitting opponent and appropriately ended the match with a crunching forehand winner down the line.

"I felt very solid today, and I think that's important against a player who can have big weapons," said the Canadian world number seven told reporters. "I kind of told myself to be ready for anything."

Later on Monday, second-seeded Russian Maria Sharapova was due to play fellow former world number one Victoria Azarenka of Belarus in an eye-catching evening match.

Azarenka has a career 7-6 advantage over Sharapova but has dropped to 38th in the world rankings after struggling for much of last year with a foot injury.

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