Fernando Alonso says he started the Mexican Grand Prix out of 'respect' for the fans despite knowing his McLaren-Honda was unlikely to last beyond the opening lap.

Another low point for the struggling team, after Jenson Button failed to make it out for qualifying, it was Alonso's turn to hit troubles ahead of the race as McLaren discovered a MGU-H problem overnight.

With no time to replace it, Alonso went into the race knowing he would probably retire after only a few corners, but the two-time world champion says he was determined to at least line up on the grid rather than simply withdraw.

"We spotted the MGU-H problem last night and this morning it was confirmed," he said. "There was no time to replace it anymore so the possibilities were not to start the race or do as much as we could knowing that maybe one lap was the maximum. For the respect of the fans, we tried to race. The first lap was enjoyable... for the first three corners."

Leaving for the airport before the race had finished, Alonso was frustrated by his seventh DNF of the year, particularly given the vast crowds that had packed the stands.

"It is a shame. One of the best races of the year and unfortunately we could not even participate. Hopefully my colleagues put on a good show because the fans are amazing."