Mercedes have insisted they must continue "raising the bar" into the second half of the F1 season to stay ahead of Red Bull and Ferrari.

The world champions' quest for an unprecedented sixth consecutive world title double appears firmly on course as the campaign prepares to resume at Spa this weekend.

The team lead Ferrari by 150 points in the Constructors' Championship, while Lewis Hamilton is 69 points ahead of the first non-Mercedes driver - Red Bull's Max Verstappen - in the standings.

But after being increasingly challenged for pole positions and race wins in the thrilling series of races before the summer break, Mercedes say they must keep improving to maintain in control.

"We're leading in both championships, but it doesn't feel that way," said team boss Toto Wolff.

"In the last few years, we saw teams perform very strongly after summer shutdown, so now we need to keep pushing.

"In that respect, we're approaching the start to the second half of 2019 more like the start to a completely new season - once again, we need to make sure that we leave no room for error and keep raising the bar."

Spa, with its legendary sequence of flowing corners, has generally suited Mercedes' cars in the current hybrid engine era - but race day has not always gone their way, with defeats to Red Bull and Ferrari in 2014 and 2018 respectively.

"We're very excited for the upcoming months with historic races like Belgium and Italy followed by the Singapore night race and many other fantastic events," added Wolff.

"The next stop takes us to Spa, one of the most iconic circuits on the calendar. Finding the right set-up is tricky because the track has a challenging mix of long straights, a wide range of corner speeds, and also considerable elevation changes.

"The weather can also be somewhat unpredictable in the Ardennes, which can make the sessions very interesting. We're looking forward to the fight."