Slovak citizen visiting Hong Kong and Beijing for up to 72 hours

Hi, I am Slovak citizen and I'll be going for business trip to Hong Kong and it will be covered by company, including fly tickets. I would like to visit friend in Beijing, but would like to avoid getting visa and use 72 hours free transit. Is it possible to do it this way in case I buy separate fly tickets from Hong Kong to Beijing and back to Hong Kong, from where I'll be leaving back to Europe ? Or do I need to buy tickets Europe - Hong Kong - Beijing - Europe ? Please let me know, thank you in advance.