The badminton star started training with the Indian legend at the end of August.
Last week, the 28-year-old was forced to concede her second round match against Thailand's Supanida Katethong at the French Open.
Sindhu pocketed the first game 21-18 and the second game was level 1-1 before the Indian felt discomfort on her left knee.
Sindhu quelled a strong challenge from Indonesia's Gregoria Mariska Tunjung.
World No. 12 Sindhu and Marin, currently ranked sixth, had earlier talked about camaraderie between them but they showed none of it on the court on Saturday.
The two-time Olympic medallist displayed her immaculate retrieving skills and exemplary net play to outshine Katethong.
Sindhu d a gritty show to bounce back from a game down to outwit Tunjung.
The eighth seed went down fighting to fifth seed Zhi Yi Wang of China.
The eighth seed will take on world No. 11 Wang Zhi Yi for a place in the final.
However, it was the end of the road for Kidambi Srikanth and Kiran George in men's singles.