It's "game on" for the top four richest men on the Chinese mainland.

Real estate guru Wang Jianlin, the wealthiest person in the country last year, announced on Friday a 5 billion yuan ($813 million) joint investment in e-commerce with Internet magnates Ma Huateng and Robin Li, the second and third richest.

They are targeting online shopping giant Alibaba Group Holding founder and chairman Jack Ma, who climbed to the top of the rich list on Thursday with a net worth of $21.8 billion, according to the Bloomberg Billionaires Index.