A recent patent filed by Mazda has once again sparked rumours of a potential RX-9 rotary sports car - which could, some reports suggest, debut at this year's Tokyo motor show in October. (Even if Mazda's CEO claims no such car is happening...)

The document details a door that opens out and upward, helping occupants to avoid hitting curbs in a low-riding car. While the idea itself isn't a new concept, Mazda's filing includes a more simplified mechanism to make closing the door easier.

Many manufacturers, particularly those that produce high-end sports cars such as McLaren, use gas struts to hold a vertically-hinged door open. This is a method labelled by Mazda as "relatively large-scale, heavy and expensive".

Instead, the company's design employs a torsion bar to hold the door in place when fully open. Mazda says the "torsional energising force" in the bar will make it easier to close the door.

A similar design is used by British marque Aston Martin, though Mazda's version creates a 15-degree angle, compared to Aston's 12.

It's reported this door opening design will make its production debut on the market version of the 2015 RX-Vision concept (below), said to be the spiritual successor to the iconic RX-7 and the more recent RX-8 coupes.

Power is expected to come from a new SkyACTIV-R turbocharged rotary engine, which was confirmed to be under development when the RX-Vision was revealed at the Tokyo motor show two years ago.

Should an 'RX-9' become a reality, it will reignite the Japanese sports car battle, taking on the upcoming Toyota Supra, the rumoured successor to the Nissan 370Z, along with the rumoured Honda 'baby NSX' - which could go by 'ZSX' according to European patent documents.

More information should come to hand in the lead-up to the Tokyo motor show in November. Stay tuned to CarAdvice for our coverage.