Fernando Alonso will take his "demanding" schedule, coupled with the predictable nature of Formula One, into consideration when he plots his future.

The two-time world champion is splitting his time this year between his duties with McLaren and racing in the World Endurance Championship, where he competes for Toyota.

Alonso, who skipped last year's Monaco Grand Prix to race the Indy 500, triumphed at the Six Hours of Spa last weekend to record his first race victory since 2013.

However, the Spaniard has struggled to compete with the leading contenders in F1 this season, leaving him "sad" at the state of the sport as he contemplates his next career move.

"I'm attempting two world championships at the same time," Alonso told the media ahead of the Spanish Grand Prix at the Circuit de Barcelona-Catalunya.

"I'm happy with the progress we did here at McLaren and the direction things are going so I'm happy here. But it's quite demanding - especially the F1 calendar.

"The biggest thing here is how predictable everything is. We can put in the paper now what would be the qualifying here on Saturday, what it would be in Monaco, what it would be in Silverstone.

"So that's something you need to take into account for future decisions. This is sad in a way for Formula 1, the direction everything went."

Alonso has yet to reach the third stage of qualifying this season but has still picked up 28 points, leaving him in sixth position in the overall standings.

He remains hopeful McLaren can still close the gap on Ferrari, Mercedes and Red Bull, the three leading teams in the constructors' championship.

"Normally Spain brings a lot of updates for all the teams, some of them work better some of them need a little bit more time, so hopefully we can benefit a little bit from that as well," he said.

"There's still a long way to go for us but at the same time we were here last year with zero points and now we are sixth and fourth in the world championship so in a way it's been a very good start. So let's keep that momentum."