SINGAPORE, Sept 30 (Reuters) - S&P 500 index futures lifted 0.3% during the first U.S. presidential debate as both candidates landed blows in early exchanges. S&P 500 futures last sat at 3,343, about 0.2% above the underlying S&P 500’s Tuesday close of 3,335.47. Investors are closely watching the debate to gauge how Democrat Joe Biden and Republican President Donald Trump are shaping up ahead of the Nov. 3 election. Dow Jones futures also rose 0.3% and Nasdaq 100 futures rose 0.2%. (Reporting by Tom Westbrook; Editing by Jacqueline Wong)