Fittipaldi -- grandson of former world champion Emerson - crashed at the high-speed Eau Rouge during a qualifying session, losing control mid-way through the corner and going straight into a tyre barrier. Fittipaldi, the winner of last year's Formula V8 3.5 championship, was taken to Centre Hospitalier de la Citadelle de Liege. His left leg suffered a complex fracture according to FIA.

Fittipaldi, 21, is balancing WEC duties with Super Formula and IndyCar this year; he had been due to make his debut at the Indianapolis 500 later this month with Dale Coyne Racing.

The accident overshadowed Fernando Alonso's first performance in an official World Endurance Championship session.

The WEC field had been qualifying for this weekend's Six Hours of Spa, the first race in the series' 'super season' which runs through until next year's Le Mans 24 Hours. Spa is set to be the debut for Alonso, who has been cleared by McLaren to race for Toyota at every WEC event in 2018 as they do not clash with F1 races.

Alonso and Kazuki Nakajima, one of his teammates in the No.8 Toyota crew, qualified second on the grid for the Spa race. The pair will be joined in the race by former Formula E champion and Red Bull junior Sebastien Buemi. The trio will also contest the Le Mans 24 Hours together on June 16/17 this year.

The No.8 team were beaten to pole position by their teammates in the No.7 Toyota, led by ex-F1 driver Kamui Kobayashi and Mike Conway.

Fernando Alonso will line up on the front row for his World Endurance Championship debut. Dean Mouhtaropoulos/Getty Images

After racing in Spa, Alonso will travel to his home F1 race at Barcelona, host of the Spanish Grand Prix. That race weekend will see Alonso pass an unenviable milestone; Saturday will mark five years since he last won an F1 race.

The Spa effort will also mark the first time he will line up on the front row of a grid since the 2012 German Grand Prix. The Spaniard was full of praise for Toyota's dominant performance afterwards.

"In qualifying, to get the lap together with no mistakes, no problems, obviously with the red flags and things like that, it's always difficult to get the full focus on it," Alonso said.

"We will start on the first row of the grid. It's a long time since I had that! It's a superb performance. The team is just amazing. One-two is a perfect start. We want to convert this in tomorrow's race."