Diniz, born in Brazil, followed up her surprise win in Basel with victory the World Cup Grand Prix in Zurich on Sunday - both on her horse Lennox.
The second win - following a four-horse jump-off against the clock - assured her place at the final in Gothenburg.
"I cannot find the right words to express my feelings," Diniz said after the Zurich Grand Prix.
She now sits in fifth place on the overall standings with 61 points - already enough for a place in Gothenburg with two shows still to go.
Diniz, 40, comes from a rich horse riding background. Her mother was a dressage champion in Brazil for eight consecutive years and her two brothers are polo champions.
She was crowned junior champion in Brazil before moving to Belgium to join one of her fellow riders Nelson Pessoa. Since then she has made a name for herself on the World Cup circuit and given her recent form, will be a serious contender in Sweden.