South Korea lagged behind in closing the gap in its economic performance and the pace of its economic growth with other advanced economies, a report showed Tuesday, indicating that Asia\'s fourth-largest economy is stuck in the \"middle-income trap.\"    South Korea ranked 26th among the surveyed 100 countries as of end-2011 in terms of an index measuring how large the gap is between the top economic power\'s economy and other countries, according to the report by the Center for Economic Catch-up. The index, calculated based on a country\'s income level, the size of its economy and per-capita GDP, measures the quantitative level of its economic catch-up, according to the report. Among emerging countries, Singapore ranked sixth, followed by China in eighth place and Taiwan at 23rd. The U.S. topped the list, followed by Japan in fourth place and Germany in the fifth spot. The report said South Korea is feared to enter into the \"middle-income trap,\" which indicates that the country has achieved a certain level of economic expansion and income but is stuck at a certain level. The report also showed that Korea is seeing its economic vitality increasingly lose luster. In terms of an index gauging the average speed of economic catch-up, South Korea ranked 56th among the surveyed countries. China stood in eighth place, followed by Brazil at 42nd and Mexico at 53rd, the report said. The indicator measures how fast a country is catching up to the No. 1 country in terms of per-capita GDP and its GDP share against the global economy. When a country enters the rank of an advanced nation, its pace of economic catch-up usually tends to slow. As of 2011, South Korea\'s GDP accounted for 1.61 percent of the global economy, down from the 1.87 percent at the end of 2005. During the cited period, the weight of China\'s GDP against the global economy rose to 10.54 percent, from 4.99 percent. Brazil and Indonesia also saw their GDP share per the global economy almost double in the cited period. South Korea is feared to enter the trend of low growth as the country\'s economic performance is lagging behind its long-term potential growth of some 3.8 percent. The country is widely expected to grow around 2.6 or 2.7 percent this year after advancing 2 percent in 2012, the slowest gain in three years.