Becker, whose BCI company recently took over the running of the Hamburg Masters Series, lost millions previously when his Sportsgate Marketing firm went bust.
Lawyers acting for the German sportsman were believed to be confident a guilty plea would see their client escape with a fine and suspended sentence.
But German state prosecutors are seeking a three-and-a-half year jail term.
Becker has admitted living in Germany, while claiming to reside in Monaco between 1991-3 in a bid to avoid paying EUR1.5m in German taxes.
State prosecutor Matthias Musiol said on the first day of a court trial Becker had evaded taxes worth millions of German marks and therefore was guilty of severe tax evasion.
Former tennis ace Boris Becker faces the very real prospect of jail in his tax evasion case, despite pleading guilty.
