Romain Grosjean has tipped Renault to come on strong again before the end of the 2015 F1 season, despite its much maligned start to the year.

The engine supplier has come in for criticism from primary partner Red Bull for its failure to get on top of the latest set of regulations, with its current V6 hybrid power unit down on speed and reliability compared with its Mercedes and Ferrari counterparts.

Despite this, Lotus driver Grosjean - who had only previously raced with Renault power until his team's switch to Mercedes in 2015 - is certain the firm has what it takes to claw back ground, before the year is out.

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"I thought they would improve a little more than what they did, especially in race one in Melbourne," he told Crash.net. "I still think they can surprise people by the end of the year.

"They've always been capable. I don't see why they wouldn't be able to find out the solutions of V6. V12 was good, V10 was good, V8 was good, so why would that not be the case...?"

Despite the shortcomings of the power unit, Red Bull remains fourth in the constructors' standings, while Toro Rosso rookies Carlos Sainz and Max Verstappen have shown flashes of form in their debut