Swiss watchmaker TAG Heuer says it is severing ties with Maria Sharapova as the world’s highest-paid female athlete started to count the cost of a failed drug test and likely ban from tennis.

Sports firm Nike and German luxury car maker Porsche also said on Tuesday they were suspending their relationship with the five-times Grand Slam champion as the 28-year-old Russian awaits a decision on whether she will be banned.

The failed drug test at January’s Australian Open, one of four annual Grand Slam events, will be costly for her at a time when sports bodies and sponsors are taking a tough line following a series of corruption and doping scandals.

Sharapova earned $US29.7 million ($A39.74 million) last year, Forbes magazine reported, and most of it came from endorsements, appearances and royalties rather than her victories on court.

Sharapova, who lit up women’s tennis when she won Wimbledon in 2004 as a 17-year-old and is still ranked among the top players, announced on Monday she had tested positive for meldonium, which is used to treat diabetes and low magnesium.

She said she had been taking the substance for a decade for health reasons and had not read an email informing her that a ban on its use in sport, imposed by the World Anti-Doping Agency (WADA), had come into force on January 1.

She will be provisionally suspended from playing tennis from March 12 and could be prevented from competing for Russia at the Rio de Janeiro Olympics this year.

Sharapova’s deal with Tag Heuer had expired at the end of 2015, and the company had been in talks to extend the collaboration, it said on Tuesday.

“In view of the current situation, the Swiss watch brand has suspended negotiations and has decided not to renew the contract,” TAG Heuer, a unit of French luxury goods group LVMH, said in a statement.

Porsche, a division of Volkswagen, said it was suspending Sharapova’s role as its brand ambassador.

Sharapova, who lives in the United States, is the seventh athlete in a month to test positive for meldonium.

She said her family doctor had been giving her mildronate, also called meldonium, for 10 years after she frequently became sick, had irregular EKG results, a magnesium deficiency and a family history of diabetes.

Sharapova competed in one tournament while using meldonium as a banned substance.

“I made a huge mistake. I know that with this I face consequences and I don’t want to end my career this way,” she said.