Defending Indianapolis 500 winner Marcus Ericsson has sent an ominous warning to his rivals in Indianapolis 500 practice, with Australia's Will Power also making an impression.

Ericsson first charged to the top of the speed chart with a lap of 229.607 mph on the 4km oval on Thursday, before turning the best no-tow speed of the day at 224.414 late in the session.

Ericsson's teammate Scott Dixon had the second-fastest overall lap for the second straight day, giving Chip Ganassi Racing another one-two finish. The six-time series champ and 2008 race winner went 229.186.

"It was a really good day for the whole Chip Ganassi team," Ericsson said. "Yesterday we worked a lot on the race cars.

"We did some changes overnight that I felt were really great. It was a very positive day and hats off to the Ganassi team."

For Ericsson, it was a significant jump from Wednesday's session when he was seventh overall and the slowest driver on Ganassi's four-car team. Tuesday's first scheduled practice was rained out.

Thursday's boost could put Ericsson in a stronger position to become the sixth driver in Indy history to record back-to-back wins, the first since four-time winner Helio Castroneves captured his first two in 2001-02.

Dixon is also chasing history. If he wins a sixth Indy pole, he will tie Rick Mears' career record and become the first driver in race history to hold the No.1 starting spot in three successive years.

Another strong showing in Indy would also help Ericsson make the case for a contract extension with one of IndyCar's top teams. He slipped from first to third in points with an eighth-place finish last weekend on Indy's road course.

"We feel strong, we feel better than last year and last year was pretty good," Ericsson said. "We feel like from what we've seen, we're going to be up there (on the starting grid) but we don't want to underestimate the other teams."

Castroneves was 17th as he attempts to earn a spot on the 33-car starting grid and seek a record-breaking fifth 500 title. Simon Pagenaud, Castroneves' teammate with Meyer Shank Racing, had a more promising day by winding up third on the leaderboard at 228.681.

Team Penske's Will Power, the 2018 race winner, was fourth and had the fastest Chevy of the day at 228.577, while Colton Herta led Andretti Autosport at 228.240, just behind Power.

The threat of rain interrupting the speedway's traditional Fast Friday festivities, or washing it out completely, made for a busier session than usual.

Thirty-four cars turned more than 3,100 laps over six hours on a warm, sunny day with a light breeze. The forecast is calling for similar conditions in the qualifying rounds set for Saturday and Sunday.

Speeds are expected to take another big jump when series officials add extra power to the cars for Friday's practice and qualifying weekend. The race is scheduled for May 28.