Felipe Massa has warned teammate Lance Stroll that the rookie's toughest Formula 1 challenge is now looming.

Eighteen-year-old Stroll has struggled in the sister Williams so far this year, but veteran Massa said that Monaco in a fortnight will be his biggest eye-opener to date.

"Monaco is probably the most demanding track of all," Massa, who admits that he is mentoring Stroll, told Le Journal de Montreal. "Even experienced drivers have problems there.

"The walls arrive very quickly. And the cars are wider this year but the streets are just as narrow as before.

"So it's going to be hard for Lance in Monaco. Nothing compares to it.

"But of course I'll be there to help him. The setup is crucial in Monaco and I will share it with him. Where and how to brake is also an art that must be mastered."

Recalling his own Monaco debut as a youngster in 2002, the 36-year-old admitted: "Like many others in their first time, it was catastrophic for me even if I did the 13th time.

"I was constantly avoiding contacts with the wall or other cars and my race ended abruptly."

Indeed, even F1 teen sensation Max Verstappen has had a tough time at Monaco in the last two years.

"That's one example among so many others," said Massa. "Many drivers hit the wall trying to do too much, not only Max. Even the veterans are not immune."

Massa also admitted that Monaco has not been a happy hunting ground for Williams.

"Honestly, it's always been terrible for our team," he said. "Don't ask me why, I don't know. It's a mystery to me."

Williams are sixth in the constructors' standings ahead of the race in the principality.