Nike has suspended its relationship with Maria Sharapova after the five-time Grand Slam tennis champion admitted failing a drug test.

Also, Swiss watchmaker Tag Heuer cut its ties with the tennis star.

Nike said it was "saddened and surprised" at her admission that she tested positive for a banned substance at the Australian Open in January.

Sharapova said she had been taking meldonium since 2006, on the advice of her family doctor.

ALSO READ: Maria Sharapova failed drugs test at Australian Open

Nike 'monitoring situation'

Her relationship with Nike dates back to when she was 11 years old.

"We have decided to suspend our relationship with Maria while the investigation continues," Nike said.

"We will continue to monitor the situation."

In 2010, the 28-year-old Russian tennis player signed a new eight-year contract with the US sportswear giant worth $70m (£49m) as well as a cut on sales of her own branded clothes.

Tag Heuer was in talks to extend its deal with Sharapova, which ran out at the end of last year.

However, Tag Heuer said those talks have now been suspended and the company has decided not to renew the contract.

Sharapova is the world's highest paid female athlete after earning nearly $30m in 2015 from winnings and endorsements, according to Forbes. These include contracts with Evian, Tag Heuer, Porsche and Avon.

In 2014, Porsche named her as its first female ambassador and she signed a three year deal with the car company.

That contract is due to end at the end of this year.

A Porsche spokesman said it was likely any personal appearances would be postponed for the time being, but that no decision had yet been taken on their long-term relationship.

Sponsors 'burned'

Sharapova, who lives in Florida, is also the face of Avon perfume Luck.

Paul Swangaurd, from the Warsaw Sports Marketing Center at the University of Oregon, said Nike's decision reflected a "new era" for the way sponsors deal with these issues.

Nike is taking a "very proactive approach", which resulted from "them being burned by a lot of athletes over the years, and growing impatient with putting so much investment behind athletes that potentially comes back to bite them in the court of public opinion".

Last month, Nike dropped Manny Pacquiao after the boxer said homosexual people were "worse than animals".

The company also severed ties with cyclist and drugs cheat Lance Armstrong as well as athlete Oscar Pistorius, who killed his girlfriend.

Failed test

Sharapova tested positive for meldonium, a substance she said she had been taking since 2006 for health issues.

The International Tennis Federation (ITF) said she would be provisionally suspended from 12 March.

Sharapova said: "I did fail the test and take full responsibility for it."

She said she had taken meldonium after being given it by her family doctor and had known the drug by the name mildronate.

"A few days ago, after I received a letter from the ITF, I found out it also has another name of meldonium, which I did not know," she said.

What is meldonium?

It is meant for angina patients but athletes like it because it helps their endurance and ability to recover from intensive exercise.

It is on the banned list now because Wada started seeing it in lots of samples and found it does have performance-enhancing properties.

It was on Wada's 'watchlist' for over a year and added to the banned list on 1 January.

Made in Latvia, it is widely available - without prescription and at low cost - in many east European countries, but it is not licensed in most western countries, including the United States.

It is thought that hundreds of athletes have been using it and there are a lot more cases in the pipeline.