Valentino Rossi says he is happy to come away with a last-gasp fourth place finish in the Indianapolis MotoGP, from a season worst ninth on the grid, but admits he needs to understand why he struggles to push on fresh tyres at the start of a race.

The Italian had made meagre progress in the early stages of the grand prix, holding down a fairly lonely seventh during the middle stages.

However, despite at one stage slipping five seconds behind the battle for fourth between Cal Crutchlow and Alvaro Bautista, Rossi's pace increased as the race entered its latter stages to swiftly hunt down the feuding pack in front of him.

Engaging in a fierce tussle with Bautista and Crutchlow over the final laps, Rossi would pounce on the latter through the last bends to secure a well-received fourth place finish.

"With Bautista it was a good battle, but especially with Cal on the last lap, we overtook two times, that was very funny," said Rossi.

"In the end, I'm quite happy, especially about the result and especially about the second half of the race. When the tyres start to slide, I ride a lot better, like entering the turn faster.

"When the rear started to slide, I remembered to ride the bike with more oversteering, releasing the brake 30, 40 meters before and making a lot of corner speed.

"When I had the new tyre with extra grip, the rear pushed the front too much, and I was a lot in trouble. In the first half of the race, I was very slow. But the second half was not so bad. Now we have to understand why, because usually the problem is the opposite.

"To go from Indianapolis with a fourth place, starting from the third row, is a good result. But if we want to try for the podium ad fight with the top three guys we have to improve."