In his second year, he won an essay competition organised by the World Health Organization (WHO) and received the opportunity to attend a healthcare workshop in Minamata, Japan, where he learned about the devastating effects of Minamata disease, a neurological syndrome caused by severe mercury poisoning. This led him to specialise in neurosurgery later on, which was not a popular discipline at the time. In 1979, Sadasivan graduated with a Bachelor of Medicine and Bachelor of Surgery (M.B.B.S.), and two years later embarked on further studies at the Royal College of Physicians and Surgeons of Glasgow, becoming a Fellow of the Royal College of Surgeons (F.R.C.S.) in 1984. He trained at the Henry Ford Hospital in Detroit, Michigan, between 1985 and 1990, obtaining a diploma from the American Board of Neurology Surgery and becoming a fellow at Harvard University in 1990. He also worked at Brigham and Women's Hospital, a teaching affiliate of Harvard, and at the Children's Hospital Boston in Boston, Massachusetts. In 1997, he obtained a Bachelor of Laws (LL.B. (Hons.)) from the University of London. Balaji joined Tan Tock Seng Hospital as a consultant neurosurgeon in 1991, where he reorganised the way stroke patients were treated, arranged for the neurosurgical intensive care unit to be computerised, introduced stereotactic brachytherapy for dealing with brain tumours, and chaired the National Neuroscience Institute's planning committee. In 1994, he moved into private practice at Gleneagles Hospital, establishing the first stereotactic radiosurgical treatment system driven by a linear particle accelerator in Singapore. He also worked with medical device manufacturer Siemens to develop image-guided surgical systems. He published more than 50 scientific papers and chapters in neurology books. Balaji left the medical profession to stand as a People's Action Party (PAP) candidate in the 2001 general election for the Cheng San–Seletar division of the Ang Mo Kio Group Representation Constituency (GRC). The electoral division, helmed by the Lee Hsien Loong (who became Prime Minister on 12 August 2004), was not contested, and Balaji was declared elected to Parliament on 25 October 2001. He was regarded as one of the "Super Seven" Members of Parliament who were made officeholders upon election, and served as Minister of State for the Ministry of the Environment (23 November 2001 – 11 May 2003), Ministry of Health and the Ministry of Transport (both 23 November 2001 – 11 August 2004).