The Colombo Stock Exchange's main index closed 0.06 percent, or 2.92 points weaker at 5,039.15, after moving within a range of 5,018-5,068 points. It had gained 5.6 percent in the six sessions through Wednesday.
The central bank has dismissed S&P's assessment, saying it is incorrect and contradictory.
The finance sector index fell 0.62 percent to its lowest since June 14, with top lender Commercial Bank of Ceylon losing 0.4 percent to 100.50 rupees and development lender DFCC Bank down 1.78 percent to 110.10 rupees.
Market heavyweight and top conglomerate John Keells Holdings PLC fell 1.07 percent to 193.90 rupees a share.
Turnover was 674 million Sri Lanka rupees ($5.08 million), below the daily average of around 923 million rupees this year.
Foreign investors were net buyers of 77.3 million rupees worth of shares on Friday extending the net foreign inflow so far this year to 22.69 billion rupees.
The rupee fell for a fifth straight session, edging down to 132.80/133.00 against the dollar, from Thursday's close of 132.80/90.
Dealers said the rupee fell to 133.40 on Friday due to importer dollar demand before recovering on exporter dollar sales. It hit a record low of 133.60 on June 12.
Read More:: Reuters (Source)