Mariam Aziz, 55, is demanding compensation from Fatimah Kumin Lim, accusing her of stealing jewellery and diamonds worth more than $18m (£11m).
Ms Lim says Ms Aziz authorised her to sell some of the jewellery and use the proceeds to pay off gambling debts.
Ms Aziz was married to the sultan from 1981 to 2003.
She told the High Court in London that she gave a diamond bracelet to one of her bodyguards for safekeeping during a night out in 2008, and she did not see it again.
“I was confused and determined to look for it so I asked the whole household, all the maids, to look for it and offered a reward,” she told the court.
“For over one and a half years, I was still looking for an answer. I never reported it to the police because I was not sure if I had lost it or misplaced it.”
Ms Aziz’s lawyers told the court that Ms Lim had arranged the sale of the bracelet to a Geneva-based company for more than $5m. They are suing for compensation for the full amount.
They also accuse Ms Lim of selling two diamonds worth more than $13m. Ms Aziz is trying to recover the stones from another Swiss firm.
The court heard that Ms Aziz first employed Ms Lim as her badminton coach in January, 1986.
She later became her bodyguard and travelled all over the world with her, spending time at Ms Aziz’s homes in London, Brunei and Singapore.
Ms Lim, who is based in Singapore and is not in court, has been made the subject of a worldwide order freezing her assets.