MOSCOW: Three of Maria Sharapova's major sponsors are cutting ties with the Russian tennis star after she acknowledged failing a doping test at the Australian Open.

Sportswear giant Nike, Swiss watch brand Tag Heuer and German luxury car company Porsche moved quickly to distance themselves from the five-time Grand Slam winner after she announced the positive test at a news conference in Los Angeles on Monday.

"We are saddened and surprised by the news about Maria Sharapova," Nike said in a statement. "We have decided to suspend our relationship with Maria while the investigation continues. We will continue to monitor the situation."

TAG Heuer said its deal with Sharapova will not be renewed. The sponsorship expired at the end of 2015 and discussions had been taking place on how to extend it.

"In view of the current situation, the Swiss watch brand has suspended negotiations, and has decided not to renew the contract with Ms Sharapova," TAG Heuer said in a statement.

"She's a one-woman marketing machine," said Nigel Currie, an independent British-based sponsorship consultant. "There are lots of male stars in the world, but not many female stars."

Currie said it's "unbelievable" how such a mistake could have happened since Sharapova has such a big support network, adding that it's also "amazing" how quickly sponsors react.

"They are paranoid about their image, and the slightest risk to their image, they run to the hills," he said.