Kweku Adoboli, 32, is accused of gambling away £1.5billion of the Swiss bank's money and wiping £3bn off its share price while working at its London headquarters.
Prosecutors claim the trader ignored financial controls, including trading limits, and invented phantom transactions to disguise his 'naked gambling' from bosses in order to boost his ego and career prospects.
It is alleged he was 'intimately aware' of how UBS's trading controls worked after moving from the back office to trading floor and was able to play the system a result.
But Adoboli hit back at allegations he took advantage of his knowledge of the bank's inner workings today, saying he just wanted to 'do my job to the best of my ability'.
He said: "It was the ability to understand the interaction of systems and accounting processes that allows you to do your job.
"It is not fraudulent; it is finding a way to do your job."
He told the court he did not believe his 'rare skillset' had made it 'easier' for him to exploit the bank.
"The reality is systems and processes evolve, number one.
"Number two, the reality is every single trader, and salesperson, trade support advisor, and settlement person has a duty to understand the system, the tools that we use.
"So I do not think it is fair to say that my experience in settlement and trade support enabled me to more easily do something that has been characterised as fraud.
"Those experiences enabled me to do my job to the best of my ability."
In an at times emotional start to his evidence Adoboli wept twice when asked about his father, who sat in the public gallery, and his 'mentor' at UBS Mike Foster, the former head of the ETF desk.
Mr Justice Keith told him he should not be embarrassed as it gave the jury a chance to 'see the man behind the name'.
Adoboli went on to describe Mr Foster, who left UBS before the period in which Adoboli is alleged to have begun his fraudulent activities, as having a 'lot of strong relationships' and being 'good at managing' both him and his desk colleague John Hughes.
After Foster left, Hughes took over as 'supervisor' of the ETF desk, said Adoboli, leaving two young men in charge of a '$50bn book'.
Hughes, who gave evidence earlier in the trial, was described by Adoboli as 'quite maverick' and 'gung-ho'.
He said: "He did not really care about the book as a whole. He cared about being profitable as a trader."
Adoboli worked in the back office at UBS from September 2003 until December 2005 when he was employed as a trader.
In September 2006 he was moved to the global synthetic equities division on the exchange traded funds (ETF) desk buying and selling ETFs, which track different types of stocks, bonds or commodities such as metals.
At the start of today's hearing two additional charges of false accounting were added to the indictment.
Mr Justice Keith, outlining what he described as the 'sensible reasons' for adding the charges, told the jury: "It's been decided to add two additional counts to the indictment.
"Those additional counts do not add to the allegations which have already been made against Mr Adoboli.
"They have been inserted to clarify the allegations which are being made."
Adoboli, of Whitechapel, east London, denies two counts of fraud by abuse of position and four counts of false accounting.