The most matches won in a year before a loss was 17*, achieved by Sampras in 1997. The American won his first three tournaments at the Australian Open, San Jose and Philadelphia that year, before losing to 43rd ranked Bohdan Ulihrach in his Indian Wells opener. Djokovic equalled that record with a win over Federer in the semi-finals and the Serbian will set a new record of 18 shall he beat Rafael Nadal and collect the title.
*Since 1990 and for those players who played both Australian Open and Indian Wells.
In my article, http://thebigtip.com.au/tennis/djoko...r-start-season
, I compare the best starts to the season (up to Indian Wells) since 1990 including Courier 1993, Agassi 1995, Sampras 1997 and Federer 2004-2007.