Mercedes express cautiousness for the upcoming Malaysian Grand Prix due to the unpredictable weather after only a fifth-place finish in Melbourne."It will be a very challenging weekend," Mercedes' team principal Ross Brawn told a press conference on Wednesday. "It is a really unpredictable environment, we wish we have a more normal weekend."The Formula 1 drivers are likely to face high temperature and unpredictable showers during the weekend in the Sepang international circuit.That makes a good start more important in the race, said Brawn."I will be very disappointed if we are not finding ourselves in the top six at the qualifying," he said.Lewis Hamilton was fifth last week in Melbourne in his first race for Mercedes, after tyre degradation forcing him to give up the two-stop plan.Hamilton's teammate Nico Rosberg retired on the 27th lap due to electrical failure, despite a decent performance in the first and second qualifying sessions.Brawn said the Mercedes cars have shown encouraging signs, but the team is still finding the best balance for the car."It's healthy for us to assume there's a gap...you need a few races to make the judgment, but it's obvious that we have a gap to close and we are working to close it."Brawn said he was very disappointed by Rosberg's early retirement.Meanwhile, Hamilton said he was happy to be with Mercedes after finishing his career at McLaren.When asked about the poor performance of his former team in the first race of the season, the 28-year-old British said he believed that McLaren would make a comeback.