Felipe Massa says he doesn't know why he struggled so much with his tyres as he finished the Spanish Grand Prix in 13th place.

Having moved up from ninth to eighth on the first lap, Massa faded throughout the race and his struggles were highlighted by team-mate Valtteri Bottas finishing fifth. Massa said he was surprised to suffer so much with a lack of grip after being competitive up until the race and has urged Williams to find out what the cause was.

"It was a very difficult race," Massa said. "I was in traffic most of the race and struggling with tyre degradation. Honestly I was struggling with overall grip in the car. It was much worse today compared to yesterday and Friday on the long run; I did a very good long run on Friday on those tyres and today was a disaster.

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"It was very difficult to make the tyres work. The only difference is that the temperature went down. Maybe it was something that it changed for me in one direction and change for Valtteri in another direction! I don't know, but it's something we need to analyse and understand because I was very competitive all weekend. The car was great and then suddenly just for the race everything changed and I was struggling. I tried a different strategy to see if it would be better but it was not."

However, Massa was keen to stress that the overall picture was positive having seen Bottas score such a strong result.

"The team is doing a very good job to improve the car and make things better race-by-race. I think that's positive. There's still a lot to do; we're pushing hard but I was really convinced that maybe this race I wasn't happy about the performance because you need downforce and we don't have so much downforce compared to other teams, but the car was more competitive than I expected.

"Now it's Monaco where we need even more, so we'll see how it's going to be there, but I really think things are going in a good way and a good line. I am convinced that everything can happen. Today we passed McLaren in the constructors' championship thanks to Valtteri's race and we were more competitive than Force India today and they are only 11 points in front. So everything is completely open and we have to fight for it."