The United States and China are the two largest economies globally in both Nominal and PPP methods. US is at the top in nominal, whereas China is at the top in PPP since 2017 after overtaking the US. As of 2021, both countries together share 41.89% and 34.75% of the entire world's GDP in nominal and PPP terms, respectively. The GDP of both countries is higher than the 3rd ranked country Japan (nominal) and India (PPP) by a huge margin. Therefore, only these two compete to become first.
As per projections by IMF for 2021, United States is leading by $6,033 bn or 1.36 times on an exchange rate basis. The economy of China is Int. $3,982 billion or 1.18x of the US on purchasing power parity basis. According to estimates by World Bank, China's gdp was approx 11% of the US in 1960, but in 2019 it is 67%.