By Abhishek Takle

SPA-FRANCORCHAMPS, Belgium, (Reuters) - Max Verstappen was fastest on the opening day of practice for the Belgian Formula One Grand Prix ahead of Red Bull team mate Daniel Ricciardo as world champions Mercedes chose to focus on race preparation.

Verstappen set a benchmark time of one minute 48.085 seconds, 0.256 quicker than Ricciardo on a hot and sunny day at the Spa-Francorchamps circuit. That was faster than Nico Rosberg’s session-topping time in Friday’s morning session which he set while trialing the ‘halo’ cockpit safety device on his Mercedes.

The German was only sixth-fastest in the afternoon, his best effort 1.076 seconds slower than Verstappen’s. Championship leader Lewis Hamilton, second in the morning, ended the day 13th and 1.697 seconds off the pace.

The Briton leads Rosberg by 19 points in the standings and heads into Sunday’s race aiming to become only the third driver ever to get 50 career wins. But he is set to start near the back of the grid after Mercedes fitted two fresh power units to his car ahead of each of Friday’s sessions. Like Rosberg, he spent the afternoon focused on long-run race simulations with both Mercedes lapping slower than their morning best. Nico Hulkenberg ended a strong opening day for Force India, running third-quickest ahead of Ferrari’s Sebastian Vettel. His team mate Sergio Perez, fourth in the morning, ended the day fifth. Fernando Alonso made up for a lack of mileage in the opening session by going 12th-fastest in the afternoon.

The Spaniard completed only three laps in the morning and was unable to set a time after the updated Honda engine fitted to his McLaren sprung a water leak.

Like Hamilton, he will start near the back of the grid, after the Woking-based team opted to fit his car with a fresh power-unit. French teenager Esteban Ocon, making his Grand Prix debut this weekend with backmarkers Manor ended the day 21st. He was quicker than fellow Mercedes-backed team mate Pascal Wehrlein in the morning but the German, who has been with Manor since the start of the season, regained the initiative in the afternoon.

A number of drivers tested the ‘halo’ cockpit protection system in the opening session. Teams are trying it out ahead of the delayed introduction of some form of cockpit protection in time for the 2018 season.

(Editing by Ed Osmond)