Spanish Grand Prix, 2nd Practice: Lewis Hamilton on top with Red Bull close behind

Formula One world champion Lewis Hamilton returned to the top of the timesheets in second Spanish Grand Prix practice on Friday, with the charging Red Bulls of Daniel Ricciardo and Max Verstappen hot on his heels.

The Briton, who leads the championship by four points from Ferrari`s Sebastian Vettel after four races, lapped the Circuit de Catalunya in one minute 18.259 seconds on the soft tyres.

Australian Ricciardo, whose car has some eye-catching aerodynamic upgrades for the start of the European season, ended the session 0.133 seconds slower after crashing in the morning while Verstappen was third.

The two Red Bulls collided in Azerbaijan two weekends ago and are under close scrutiny by team bosses who could intervene if their rivalry risks overheating again in Sunday`s race.

Vettel, who will be chasing his fourth successive pole position on Saturday, was fourth and Hamilton`s team mate Valtteri Bottas fifth.

Vettel`s team mate Kimi Raikkonen suffered a power unit problem.

Bottas set the day`s overall fastest lap in the opening session with a time of 1:18.148 seconds on the soft tyres, a remarkable 0.849 faster than Hamilton who won from pole in Spain last year.

Hamilton has not felt fully comfortable with his car and recognised that his championship advantage owes a lot to luck.

"At the moment, I am punching below my weight and that is not sufficient to win a world championship," the four-time champion had told reporters on Thursday.

"I have zero comfortability... I`ll take it for now but in the long term l can`t continue to rely on it."

The virtual safety car was deployed in the morning when Ricciardo`s car ran across the gravel and into the barriers.

"Thankfully, the damage is pretty light," team principal Christian Horner told Sky Sports television. "It was the old front wing so he`s a lucky boy."

Home hero Fernando Alonso started the weekend a morale-boosting sixth, behind Raikkonen, with his McLaren displaying a heavily-revised three-pronged nose -- one of several conspicuous upgrades around the pit lane.

The Spaniard later slipped back to 12th.

Ferrari meanwhile caught the eye with wing mirrors mounted on the halo head protection device as an experiment.

Williams had another trying session as Polish reserve Robert Kubica took part in a race weekend for the first time since 2010, three months before he suffered serious injuries in a rally crash.

The former race winner was 19th, in 1:21.510, and faster than teenage Canadian team mate Lance Stroll, who was in last place on the timesheets with a best of 1:22.756 after his car went off into the gravel.

Kubica`s car had a newer floor than Stroll`s, however.

"Unbelievable how bad the balance is," Stroll complained over the radio.

Kubica was replacing Russian rookie Sergey Sirotkin, who returned for second practice and was slowest in 1:22.060.