Muguruza, the reigning Wimbledon champion, appeared on cruise control at first at the Indian Wells Tennis Garden in an event that is a Women's Tennis Association Premier Mandatory tournament.

The Spaniard won the first set 6-2 and led 3-0 in the second before the tables were turned abruptly.

Vickery claimed 13 of the next 16 games to take the next two sets 7-5, 6-1 in a clash lasting more than two hours. 

"The atmosphere really helped me even though I was feeling it a bit physically," the winner said.

"I kept telling myself, 'Just do your best'."

Great Britain's Wimbledon semi-finalist Johanna Konta also suffered a surprise loss today as her poor start to the season continued.

The world number 11 lost 7-6, 6-4 to 54th-ranked Czech Republic's 18-year-old Markéta Vondroušová in their second round clash.

World number one Simona Halep of Romania enjoyed a more successful day than her fellow seeds, beating another Czech player, Kristýna Plíšková, 6-4, 6-4.

Latvia's French Open champion Jeļena Ostapenko recovered from a second-set setback to beat Switzerland's Belinda Bencic 6-4, 3-6, 6-1.

It was also announced today that Russia's Maria Sharapova has "mutually agreed" to part company with her coach of four years, Sven Groeneveld, after her first round loss at Indian Wells.

Top seeds are still yet to enter proceedings in the men's singles draw of a tournament which is a ATP World Tour Masters 1000 event. 

But Australia's Nick Kyrgios has withdrawn before his scheduled opening match with Russia's Danil Medvedev tomorrow due to continued pain in his right elbow.