Indianapolis - Two-time Formula One world champion Fernando Alonso endured a frustrating day of Indianapolis 500 practice on Tuesday, completing just 50 laps as he wrestled with electrical issues.

The Spaniard posted a best lap of 224.162 mph (360.75km/h), on the 2.5-mile Indianapolis Motor Speedway oval in his Chevrolet powered McLaren - 32nd-fastest out of 30.

Australia's 2018 Indy 500 winner Will Power topped the practice speeds at 229.745 mph in contrast.

Team principal Gil de Ferran told NBCSPORTS.com that it was a difficult day, all the more so since the team are "baffled" by the problem.

"Obviously, I'm disappointed and a little frustrated we had the issue in the afternoon," de Ferran said.

"We are trying our best to understand what the problem is. So far, it’s baffling. We lost the entire afternoon session."

De Ferran and McLaren had not determined the cause of the electrical issue late Tuesday as they continued to scour over reams and reams of data on their computers.

NBCSports.com was allowed into the team's garage in Gasoline Alley as lead engineer Andy Brown continued to look for the cause of the issue.

"If we understood what the issue is, it would have been fixed," de Ferran said. "That's why it's so baffling. It's an electrical issue; not mechanical.

"It's been a difficult start. No doubt about that. We'll see what we can do."

Alonso, 37, led 27 laps as an Indianapolis 500 newcomer in 2017 before a mechanical problem ended his race with 20 laps remaining.

He was named the Indy 500 Rookie of the Year for his efforts.

Alonso's career includes F1 crowns in 2005 and 2006, Monaco victories in 2006 and 2007 and a share of the 2018 24 Hours of Le Mans title from the pole alongside team-mates Sebastien Buemi of Switzerland and Kazuki Nakajima of Japan.

He's back again to try to join Britain's Graham Hill as the only drivers to have won the "Triple Crown" of Le Mans, Monaco and Indy classics, taking the Indy 500 in 1966.

In 2017, McLaren partnered with Honda and team Andretti Autosport.

This year, they are powered by Chevrolet and have an engineering alliance with the smaller Carlin Racing outfit.

"We are under no illusions how big a challenge we have," de Ferran said. 

Practice for the May 26 race continues on Wednesday, with qualifying on the weekend.