Rain started coming down in the 62nd lap of the 75-lap race and was called on the 73rd lap. Joanides trailed Thorn by milliseconds throughout the race and constantly had the front of his car positioned at Thorn's back tire.
However, with the yellow flag being thrown early, Thorn came away with the victory, taking the checkered flag.
Despite the early finish to the race, Thorn said he was just happy to come away with the victory.
"Nowadays, a win is a win," said Thorn, who raced from the pole. "Even without rain we had a good enough car. Racing's one of those things. Luck, if it's on your side, you'll do very well."
Thorn added it was a clean race with Joanides.
"We've raced together before," Thorn said. "(Joanides) raced me clean. There were a couple of times he could have been more aggressive, but he gave me respect. Every time I did well, he was beating me a little. He was better on the short run."
Thorn's best speed was 95.017 mph and his best lap time on the one-half mile track was 18.944 seconds.
In addition, driver Rod Johnson, who was racing in the Pepsi Super Late Models - a race that was washed out in the 13th lap and postponed - set a course record earlier in the day with a qualifying time of 17.571 seconds.
Johnson was leading his race when it was called because of more inclement weather.