NEW YORK (AFP, REUTERS) - US sportswear maker Nike said Monday it was halting its relationship with Maria Sharapova, after the tennis star announced she had tested positive for a banned substance.

"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."

In 2010, the tennis star signed one of the most lucrative sponsorships by a sportswoman, an 8-year deal with Nike worth almost US$70 million (S$98.8 million).

Earlier, the Russian former world No. 1 announced at a press conference that she had tested positive for meldonium, a substance she has been taking for the last 10 years for health reasons.

Meldonium became a banned substance earlier this year, something the five-time Grand Slam champion claimed she had no knowledge of.

"I had been taking this medicine for the past 10 years, but on Jan 1, 2016 this became a prohibited substance which I did not know", she added.

She said she was prescribed the drug in 2006 after suffering from irregular heartbeat and displaying early signs of diabetes.

"I received an email on 22 December from Wada (World Anti-Doping Agency) about the changes happening to the banned list and you can see prohibited items, and I didn't click on that link."

She is the seventh athlete in a month to test positive for meldonium, which is used to treat diabetes and low magnesium, and was only banned by the World Anti-Doping Agency as of Jan 1.

The ITF's anti-doping program calls for a four-year suspension for a positive test, but that ban can be reduced in various circumstances, such as for first-time offences or if the player shows no significant fault or negligence. If a player bears no fault or negligence, there is no suspension.

According to Forbes, she earned $29.5 million in 2015, mostly from endorsements.