The Bombay Stock Exchange benchmark Sensex on Friday opened nearly 79 points low on selling by funds and retail investors. The 30-share index, which had gained 34.63 points in the previous session, shed 0.43 per cent, to 17,913 points with stocks of auto, banking and realty sectors retreating. The wide-based National Stock Exchange index Nifty fell by 25 point to 5,383. Brokers said, sentiments turned somewhat weak on the first session of new settlement in the derivatives segment after ONGC, Hero Honda and SAIL posted dip in their net profits for the quarter ending June 30, triggering selling by funds. Besides, weak trend on the other Asian bourses following overnight losses in the US markets also kept pressure.