Maria Sharapova of Russia in action at the Miami Open. [Photo/SIPA]

MIAMI - Maria Sharapova was embarrassed by 97th-ranked Daria Gavrilova 7-6 (4), 6-3 at the Miami Open second round on Thursday.

The early exit for the world No 2 is the biggest upset so far at the Masters tennis tournament and also the world defeat for five-time Grand Slam winner.

Sharapova lost in the first round here on her tournament debut in 2003. Thursday's defeat is her opening match in Miami after took a first round bye.

"Of course it's a bit of a surprise ... I'm expected to win," said the Russian, who has never lifted the trophy in Miami despite five trips to the final.

It's also a surprise for Gavrilova. "I still can't realize that it's my dream," said Gavrilova, who said she had dreamed of beating Sharapova ever since she saw her countrywoman beat Serena Williams in the Wimbledon final in 2004.

Sharapova lost the opening set after nearly an hour and quickly found herself trailing the former junior world No 1 4-1 in the second.

Sharapova clawed a break back but was then broken to love as Gavrilova set herself up for the win with a 5-3 lead and closed out her biggest career win on first match point.

The triumph was Gavrilova's first against a top-10 player. She had never before beaten anyone ranked higher than 35th.

In the men's competition, former US Open champion Del Potro was uspet by Canadian Vasek Pospisil 6-4, 7-6 (7) in the first round.

The Argentinean, who had come back from right wrist surgery in 2010, underwent surgery on his left wrist last March.

He finished the 2014 season outside the top 100 for the first time since 2005 after contesting just four tournaments during the year.