Red Bull Racing driver Daniel Ricciardo feels that all is not lost in his team's push to match benchmark Mercedes - despite his new RB13 Formula One car not setting the timesheets alight in pre-season testing.

"I definitely feel like we have improved each time I've left the garage and that's encouraging,"?he said.

"I feel there's still more to come and I think that's natural. We are not quite the fastest yet but we're not too far off and we've got a good base behind us."

Still, three-time defending constructors' champions Mercedes have lost none of their speed over the off-season despite Formula One's technical regulation revamp, with the cars expected to be three to six seconds faster per lap, via wider tyres and far more aggressive aero styling.

New Silver Arrows driver Valtteri Bottas set the fastest overall time at the first pre-season test; and went four-tenths quicker on Day 2 of the second test.

Still, Ricciardo says Mercedes may have less in reserve than expected.

"I think they're holding back a bit, I wouldn't say a lot,"?he said. "But that's purely a guess. I haven't really studied it, but that would be my gut feeling. Sure, they've got a bit in the bag, but I don't think they're going to blow everyone out of the water come Melbourne. Hopefully I'm right."

To be ready for his home race, Ricciardo worked out over the break, dedicating six weeks to improving his strength.

Fitness was a key focus for all the drivers this year, with the faster cars upping cornering speeds and G-forces which will make for a tougher physical challenge, and pushing them to the limit of performance.

"It does start to beat you up a bit more," Ricciardo said.

"So I think that's good, it's what we all wanted. Some races we'll finish the race in pretty good nick, but I'm sure we're going to get to some, especially the hot ones and you'll see a few of us reaching out for some water mid-race."

Of course, the drivers' toughest test will be at Marina Bay for the 10th F1 Singapore Grand Prix.

For the fans, the event tops travel wish lists worldwide with its mix of stunning night racing, big-name music acts, and iconic backdrop. But for the drivers, it's a genuine endurance test just to finish, being the longest race of the season.

Ricciardo, though, isn't fazed.

"The last few years I've put emphasis (on Singapore) with my training and my preparation, and I think the results have shown that I've been on the podium three years in a row there," he said.

"The aim is to be better prepared than everyone, and I believe that I am but it's certainly one that if a driver doesn't quite take some of those real physical ones seriously, if they're out enjoying their weekends more than they should, then I definitely feel it will show this year."

Interestingly, Ricciardo considers Singapore one of his home races, due to the city-state's proximity to his hometown of Perth.

"It's one of these iconic races now, which is pretty impressive for a track which hasn't even hosted 10 Grands Prix yet. I think hats off to them; they do it really well," he said.

"There's a lot of hype and excitement that comes with that race and I certainly enjoy it on the calendar."