Sky Sports F1 commentator David Croft was left baffled at the Chinese Grand Prix on Friday as a spontaneous grass fire forced the session to be red-flagged after just 15 minutes.

With 15 minutes on the clock and Max Verstappen and Sergio Perez leading the way on the timing tower, the stewards were forced to throw a red flag to deal with a small fire on the grass beside the circuit.

This incident caught commentator Croft off guard. "I don't think I have ever, ever seen grass catch fire by the side of a Formula One track," he exclaimed to co-commentator Nico Rosberg.

The 2016 world champion then asked Croft how he thought the fire started, to which he responded: "I would not even like to begin to speculate on that one.

"And I tell you what, that would not have happened on Wednesday because when we landed here in Shanghai there was the deluge of all deluges. It was like a wet weekend in Grimsby when we landed here!"

Thankfully for Croft, pit lane reporter Ted Kravitz was able to shed some light on the situation. He explained: "Just on the apex of that corner specifically, the cars do bottom out and they are producing sparks.

"And with the wind blowing where it is - so just downwind - I can absolutely imagine that the reason for the fire was that. There was a spark that came off the car's skid blocks and then travelled downwind a bit and then hit that patch of dry grass."

When the countdown did resume for one of the most critical practice sessions of the season, it wasn't a familiar name at the top of the standings.

Aston Martin's Lance Stroll was the pace-setter, punching in a time over three-tenths faster than second-placed Oscar Piastri, while the Red Bull duo of Verstappen and Perez followed suit.

Haas enjoyed a strong outing, taking the chequered flag in P5 and P6 but there was plenty of concern at Mercedes as George Russell and Lewis Hamilton were classified in P17 and P18 respectively.