China is still the second largest economy by almost every measure:
Emerging markets, by definition, grow at higher rates, mostly because they don't have the advanced infrastructure, technology, and abundance of developed natural resources of first world economies.
Simple math will show you that China ($12.1T), even at twice the growth of the US, will take almost a decade to catch up to the US ($19.4T) in terms of Nominal GDP. That assumes growth rates over 6.5% for China and 2% for the US.
In terms of Purchasing Power Parity (PPP) GDP, China is slightly ahead of the US, but most economists dismiss that statistic since due to its large population, China's GDP per capita is ranked 73rd in the world, so the PPP number, even though it is large, is virtually meaningless given the disparity in per capita purchasing power with most developed economies. For example, among the top economies, the PPP per capita of the US is $62,606 (10th), while the PPP per capita for China is $18,110, between the Dominican Republic and Botswana.
And yes, the Yuan was automatically designated a reserve currency by the IMF in 2016 when it exceeded 1.0% worldwide holdings, but while the US Dollar is at 61% holdings, the Yuan at 1.89% is still eclipsed by the Euro, Mark, Pound, French Franc, Yen and even the Loonie.
Countries by GDP