Pascal Wehrlein ensured that Mercedes remained at the top of the F1 pile at the Red Bull Ring as the first day of in-season testing at the Austrian venue was affected by rain.

With the entire morning session wiped out by a downpour that made running impossible, organisers decided to tack on another couple of hours beyond the scheduled end of play at 6pm local time, but the precipitation returned to make the extra offering largely meaningless.

In between, the latest class of F1 hopefuls took turns to top the times as they vied with new yardstick Max Verstappen for fastest time. Having been one of two drivers to venture out for an installation lap when conditions were at their worst, Stoffel Vandoorne opened the bidding by pushing McLaren to the rare heights of P1 as soon as running began after the lunch break, but the Dutchman was quickly relegated to a supporting role as, first, Verstappen and then the new wave of Esteban Ocon, Antonio Fuoco and Wehrlein battled over top spot.

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Having lowered the bar close to the 1min 20secs bracket, Verstappen's development programme briefly allowed his rivals to seize the upper hand, with Ocon taking the baton and running the target close to the 1min 17s as the track continued to dry out.

Once it was ready for slicks, however, Verstappen set a heavily revised target of 1m 11.946s, trimming tenths off here and there before Ocon responded with a half-second improvement to set the bar at 1m 11.192s. That mark stood until the red flag - last seen to prevent any foolhardy running before lunch - reappeared to cover the medical car's despatch to turn one.

The cause was Fuoco's second sizeable shunt at the circuit in three days, having crashed out of the GP3 race on Sunday, and once again it was a relief to see the Italian walk away, although his Ferrari was less healthy as it made the short trip back to the pits on the back of a rescue truck. Fuoco had been edging closer and closer to Ocon's time, recreating their GP3 rivalry at a higher level, before he lost control of the SF15-T and hit the tyre wall with some force.

With Fuoco out of the picture, it was time for a new threat to Ocon's top spot to emerge, and it came from Wehrlein, who posted a 1m 11.073s lap to snatch P1 with just over an hour of the original session remaining. The DTM ace then revised the target to 1m 11.005s shortly before the rain returned with a vengeance, unlike the brief flurries that had dotted the afternoon.

Although cars returned to the track in the final 90 minutes, the conditions ensured that there was no threat to Wehrlein's time and the teams took the opportunity to try wet-weather settings or practice their pit-stops.

The German thus claimed day one honours having expected to spend the hours at the wheel of a Force India rather than the Mercedes he took to top spot. His replacement at the Silverstone team, Ocon, duly took second place, a tenth off the pace, but nevertheless ahead of the closely matched pair of Fuoco - who had easily laid claim to the biggest workload prior to his accident - and Verstappen, who ended the day 3/100ths apart. Verstappen duly went on to complete close to 100 laps in what was clearly a busy - if extended - afternoon session, swapping places with Fuoco shortly before the rain returned.

Romain Grosjean, who spent a subdued day in the lower half of the order, ended it fifth overall, three-tenths off Verstappen, with Pierre Gasly, another late improver who had briefly topped the times for Red Bull in the early afternoon, finding a late gain to demote Raffaele Marciello from sixth. Marciello made a more belated appearance than any of his rivals, but recovered to sixth with a lap of 1m 11.826s in the Sauber.

The Italian was comfortably ahead of Vandoorne, with Williams' Susie Wolff bringing up the rear. While the McLaren proved reliable for its Dutch pilot, he was only able to chip away at the midfield times, and Wolff left it late to hack chunks off her early offering to eventually wind up on 1m 13.248s, just under a tenth shy of her nearest rival.

Testing continues into a second day tomorrow [Wednesday], with a nearly-new cast. Race regulars including Austrian GP winner Nico Rosberg as well as Daniel Ricciardo, Fernando Alonso, Valtteri Bottas and Felipe Nasr will all be in action with their usual teams, while Wehrlein switches back to Force India, Esteban Gutierrez takes over from Fuoco at Ferrari, Jolyon Palmer gets another opportunity with Lotus and DTM champion Marco Wittmann makes his F1 debut with Toro Rosso.