Fernando Alonso is gearing up for his landmark 400th Grand Prix this weekend, with the Spaniard believing the milestone highlights "my passion for F1".

The two-time world champion, who is ninth in this season's championship standings, will become the first driver to reach that figure at the Mexico City Grand Prix.

Even after all these years, Alonso says getting behind the wheel "pays off all the sacrifices" for the demands of travelling to 24 races a year.

"To reach 400 now is a big number," he told F1's Beyond The Grid podcast. 

"Knowing that no one reached that number in the past - maybe someone does in the future, but not many, let's say a group of five or 10 maximum - it just demonstrates my love for racing, for F1, how much I enjoy this lifestyle, motor racing in general."

World champion in 2005 and 2006, Alonso acknowledged he could never have envisaged wracking up 400 races when he entered F1 as a teenager in 2001.

And the 43-year-old's achievement is even more remarkable, 15 years after he considered walking away from the sport for good.

"I think that guy in 2001, I was not really thinking too much about the future," he added. "The dream was coming alive, driving [in] F1, the first race.

"I didn't have a clear roadmap into my career. I didn't know exactly what the next race was, what my next team would be. I was improvising [and] every weekend was a new adventure.

"What I would say is that when I won the [second] championship in 2006, and then I joined McLaren, I had a three-year contract for 2007, 2008 and 2009, and I was 99% sure that 2009 would be my last F1 season. That was my very clear plan in my head.

"When I signed that contract, in my head at that time, it was like a long-term contract and three years may feel long, but this is the last. I had already fulfilled my dream. This was beyond my wildest imagination to be an F1 champion, so what else could I do here?

"I don't think that the 19-year-old, Minardi 2001 Fernando will think something strange about the 400 Grands Prix, because I was not thinking too much into the future. But in 2007, for sure, this would be a surprise."