Alonso cruises home to win last week's Canadian Grand Prix

The defending champion has made the best start to a season in Formula One history, thanks to six wins.

But despite holding a 25-point lead over Ferrari's Michael Schumacher going into the Indianapolis race, the Renault driver said: "I know I can't relax.

"I have to keep working in every race because anything can happen."

The 24-year-old Spaniard has won four Grands Prix in succession but he knows his lead could be wiped out within three races.

"The first half of the season went well," he said. "And now I have to finish the job in the second half.

Indianapolis will be the most difficult race in the whole championship for us. Fernando Alonso

"This will be one of the most difficult races for us this weekend because of the characteristics of our car.

"I haven't finished in Indianapolis so I have to take the flag. My goal right now is to finish the race, hopefully on the podium."

Alonso never got the chance to win Indianapolis in his championship-winning campaign last season due to the infamous Michelin pull-out.

Worries over the safety of Michelin tyres forced Renault and six other teams to pull into the pits after the warm-up lap.

Now Alonso is keen to avenge that disappointment.

"Indianapolis for sure will be the most difficult race in the whole championship for us," he said.

"The car was never competitive here, we don't know why but we hope it will change. We expect a good performance here but we have some doubts."

Schumacher's excellent Indianapolis record suggests he will be the driver most likely to deny Alonso a fifth straight race victory.

The seven-time world champion has won four of the six Grands Prix at the Brickyard.

But the German insists he has no extra confidence this weekend, especially after being soundly beaten by Alonso in the last week's Canadian Grand Prix in Montreal.

"I still have a good record in Canada but it doesn't really mean anything," he said. "You have to look at the situation now."