IndyCar series leader Scott Dixon soared to the top in Thursday practice at the Indy 500 in a day of running punctuated by an accident for Fernando Alonso.

Dixon fired the #9 Chip Ganassi Racing Honda to a 39.8050s (226.102 mph, 363.875 km/h) lap around the famed Indianapolis venue in the final hour after two-time Formula One world champion Alonso found the wall.

The Spaniard still managed to set the ninth quickest time (224.363 mph, 361.077 km/h) and was the fastest of the Arrow McLaren SP entries.

Dixon, who leads the championship by 49 points through six races, was 0.4mph ahead of Takuma Sato’s quickest time set earlier in the day.

Marco Andretti, who was second on the opening day, dropped his Andretti Herta Autosport machine into third (225.249 mph, 362.503 km/h) to make it a Honda top three.

Conor Daly (225.106 mph, 362.273 km/h) sent his Ed Carpenter Racing entry to fourth and was the last driver to break the 40s barrier, ahead of rookie Alex Palou and Colton Herta.

Driving the Rick Ware Racing/Byrd/Belardi entry, Australian James Davison impressed to 10th (224.315 mph, 361.000 km/h), with countryman Will Power only 23rd (222.978 mph, 358.848 km/h).

Felix Rosenqvist and Dalton Kellett recorded the most laps of the day at 141 each.

Cars will return to the track for the Fast Friday session at 01:00 AEST on Saturday (11:00 Friday, local time).

Friday running will see the cars’ twin turbos increased from 1.3-bar (130kPa) to 1.5-bar, a tweak which will yield up to 90 extra horsepower ahead of the four-lap qualifying runs later in the week.

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