Rahal, the 19-year-old son of 1986 Indianapolis 500 winner Bobby Rahal, recovered from trouble early in the race to claim victory in the Honda Grand Prix of St Petersburg on Sunday.

It was the first victory by a former Champ Car driver in just the second race since the two open-wheel series unified under the IndyCar banner.

Rahal skipped the season opener last week at Homestead-Miami Speedway after he crashed there during practice before the race. Instead, his Newman/Haas/Lanigan Racing team chose to focus on St Petersburg, the first street race of the season.

"It doesn't get any sweeter than this," Rahal said. "I knew we had the pace and I knew if we stayed calm; if I could attack the car, we could pull away. You don't want your focus to get off the car and the race, and we never did."

Helio Castroneves had a shot at Rahal on a restart with five minutes left in the two-hour race, but Rahal beat him into the first turn and began to pull away.

The victory made Rahal the youngest winner ever in a major open-wheel race - 74 days younger than Marco Andretti was when he won at Infineon Raceway in 2006.

Following Castroneves was pole-sitter Tony Kanaan, who led early as the race started under wet conditions but fell behind as his team tried a different strategy.

EJ Viso, who also led briefly, finished third, followed by Enrique Bernoldi, Hideki Mutoh, Oriol Servia and Will Power, giving former Champ Car teams five of the first eight positions.

Castroneves, who had pitted 23 laps after Rahal's last stop, lost his chance to catch Rahal on fuel but gained a chance to catch him on a restart when Vitor Meira and Franck Perera crashed on the 76th lap. By then a timed event, the race had just minutes remaining, and Castroneves's only shot at Rahal appeared to be on a restart.

With smoke from his locked right-front tyre, Rahal outbraked Castroneves heading into Turn 1 on the restart and went on to win, becoming the fourth driver to win in his first IndyCar race.

"Good job for him," Castroneves said. "He did an excellent job. It's a great day for motor racing. It just proves that guys who execute are the guys who are successful."

Bobby Rahal, who won three CART championships during his career, was emotional in his assessment of his son's effort. "It's a tremendous moment," Rahal said. "I'm so proud of him. He drove tremendously. The car was there and he was there. It's just a great moment."

The race began with 10 laps under caution while sweepers attempted to reduce large puddles on the track following a downpour minutes before the scheduled start.

Tony Kanaan took the green flag on the 11th lap, and Ryan Hunter-Reay - who started sixth - spun behind him on the exit of Turn 12. Hunter-Reay continued, but dropped to 18th.

Minutes later, Ed Carpenter and Danica Patrick spun in separate incidents. Marco Andretti then spun, bringing out a full-course caution, and barely got restarted in front of the pace car to stay on the lead lap.

At that point, the different strategies began when the pits opened on the 16th lap. Kanaan pitted, as did Castroneves, and Justin Wilson assumed the lead.

On the 32nd lap, as Ryan Briscoe pursued Wilson for the lead, another full-course caution was ordered for debris.

Most of the leaders hit the pits, and most took on slick tyres as the groove began to dry. Briscoe stayed out and inherited the lead, but later he had to pit, giving the lead to Enrique Bernoldi.

On the 48th lap, Viso passed Bernoldi for the lead. Nine laps later, Briscoe crashed hard on the exit of Turn 10, bringing out another full-course yellow. Hunter-Reay and Graham Rahal stayed out, giving Hunter-Reay the lead, while Viso held third position.

Rahal passed Hunter-Reay heading into Turn 1 on the restart on the 64th lap, while Castroneves passed Viso for third place moments before Ed Carpenter and Wilson crashed, bringing out another caution.

On the restart, with less than 20 minutes remaining, Castroneves passed Hunter-Reay for second.

Pos Driver Team Time

1. Graham Rahal Newman Haas Lanigan Racing 2h00:43.5562
2. H.Castroneves Team Penske + 3.5192
3. Tony Kanaan Andretti Green Racing + 5.5134
4. Ernesto Viso HVM Racing + 8.8575
5. Enrique Bernoldi Conquest Racing + 9.6360
6. Hideki Mutoh Andretti Green Racing + 10.0071
7. Oriol Servia KV Racing Technology + 11.2871
8. Will Power KV Racing Technology + 12.8493
9. Justin Wilson Newman Haas Lanigan Racing + 14.3598
10. Danica Patrick Andretti Green Racing + 16.7298
11. AJ Foyt IV Vision Racing + 20.8319
12. Dan Wheldon Target Chip Ganassi Racing + 24.7800
13. Darren Manning AJ Foyt Enterprises + 45.8601
14. Jay Howard Roth Racing + 1 Lap
15. Buddy Rice Dreyer & Reinbold Racing + 1 Lap
16. Mario Moraes Dale Coyne Racing + 1 Lap
17. Ryan Hunter-Reay Rahal Letterman Racing + 2 Laps
18. Ed Carpenter Vision Racing + 3 Laps
19. Vitor Meira Panther Racing + 8 Laps
20. Franck Perera Conquest Racing + 8 Laps
21. Townsend Bell Dreyer & Reinbold Racing + 8 Laps
22. Scott Dixon Target Chip Ganassi Racing + 9 Laps
23. Ryan Briscoe Team Penske + 27 Laps
24. Bruno Junqueira Dale Coyne Racing + 39 Laps
25. Marco Andretti Andretti Green Racing + 42 Laps
26. Marty Roth Roth Racing ---

Fastest lap: Kanaan, 1:03.8874 on lap 76