(CNN) -- Tony Stewart's profession is driving for millions of dollars a year in NASCAR races, but his hobby is racing against amateurs on dirt tracks for trophies.
Stewart, 43, started in go-carts when he was just 5 in Columbus, Indiana, 50 miles south of the Indianapolis Motor Speedway.
He rose through the ranks, winning the Indy Racing League championship in 1997 before moving to NASCAR two years later.
He's won three championships in NASCAR's top series -- the Sprint Cup -- and he's a co-owner of his racing team.
Other drivers started calling him "Smoke" because that's what they saw coming from his tires as he steered aggressively through turns on asphalt tracks.
While burning rubber wore down tire tread needed late in a race, it added to his reputation as one of the most competitive drivers, both behind the wheel and off the track.
Sponsors pay big bucks to display their logos on Stewart's No. 14 car because they know millions will watch it speed around the big track hundreds of times in a single event, as many as three dozen weekends a year.
His bad-boy antics, the fights and the words, make him stand out among dozens of other personalities in the sport.
Even the fans who boo him are watching. He shows up unshaven on race days in contrast to more polished drivers who are more careful with words and actions to avoid alienating sponsors and fans.
Winning, either on NASCAR's asphalt or unsanctioned dirt, is Stewart's goal. As he sacrifices valuable tire tread for track cred, he also puts great passion into his non-paying hobby, carried out at the risk of his big-money professional driving.
"Being on a dirt track a lot of times is like being on a gravel road," Stewart said in a 2006 interview. "If you go just a little bit too fast the car's going to slide. If you hit the gas too hard, it spins the tires or kicks the rocks up. If you try to stop too fast, you skid, you lock up the brakes. That's what makes driving on dirt more technical."
When he won a dirt track race in Michigan last month, he tweeted to fans as if he'd won the Daytona 500: "If you haven't heard already, I ran my sprint car tonight and won! Felt great to run it again. Was a 2nd place car but had a little luck :)" That "little luck" was the driver of the lead car crashing.
If you haven't heard already, I ran my sprint car tonight and won! Felt great to run it again. Was a 2nd place car but had a little luck :)— Tony Stewart (@TonyStewart) July 19, 2014
1yr ago today my life changed. Thank you to everyone that worked so hard to get me back to where I'm at today. It's your life, live it!— Tony Stewart (@TonyStewart) August 5, 2014
Stewart was leading a 30-lap dirt track race at Southern Iowa Speedway a year ago when his car flipped, breaking two bones in his right leg. It ended his NASCAR Sprint Cup 2013 season 15 races early, costing him and his sponsors a lot of money.
The August 11, 2013, race at Watkins Glen, New York, was the first start Stewart missed in 521 NASCAR races since his 1999 debut at the Daytona 500. Ironically, he missed Watkins Glen again this year because of a dirt track tragedy on Saturday.
Stewart hit and killed another driver, Kevin Ward Jr., during a sprint car race at Canandaigua Motorsports Park in Ontario County, New York.
Video shows that Ward and Stewart appeared to make contact coming out of a turn and Ward hit the wall and was knocked out of the race.
Ward, 20, then got out of his car and walked down the track toward the other racers, who had slowed for a yellow flag. Ward pointed a finger and appeared to be yelling. One car swerved to avoid him, but Stewart's right-rear tire appears to hit him as he passes.
"There aren't words to describe the sadness I feel about the accident that took the life of Kevin Ward Jr.," Stewart said in a written statement.
Veteran race fans remember a rookie driver who jumped out of his car during a NASCAR race in 1999 after another driver spun his car into him in Martinsville, Virginia. That driver was Tony Stewart.
Stewart had traded bumps throughout the race with Kenny Irwin, who had been his dirt-track rival before they both moved up to NASCAR.
But the last crash against the wall disabled Stewart's car. Stewart waited on the track for Irwin to circle around under a caution flag. Stewart threw his gloves at Irwin's car, and jumped onto its right side while the car was still moving.
Stewart's run-ins with other drivers are legend, including after his car crashed with Matt Kenseth at Tennessee's Bristol Motor Speedway in 2012. Stewart waited in a driving lane on pit road for Kenseth's car to return to the track. Stewart threw his helmet at Kenseth as the car sped by. He told reporters later that he doesn't "enjoy getting mad like that."
Stewart rushed after driver Joey Logano on pit row after a Fontana, California, race last year. He was angry that the younger Logano had blocked him from passing during a restart. Logano said he was just "racing hard" because he thought he could win.
Stewart has also had his angry moments with fans and the media.
He was investigated, though never prosecuted, for an alleged assault against a female fan at Bristol Motor Speedway in 2002, the year he won his first of three NASCAR championships.
NASCAR officials fined Stewart $10,000 and ordered him to get anger management counseling for hitting a photographer after a bad race at the Indianapolis Motor Speedway in 2002.
Stewart has removed himself from racing for the present in the aftermath of Ward's death. He will not compete in a dirt track race in Plymouth, Indiana, next Saturday as he had planned, the track announced Monday.
He has not yet decided if he will race in the next NASCAR Sprint Cup event at Michigan International Speedway next Sunday, his spokesman, Mike Arning said Monday. "He is grieving, and grief doesn't have a timetable."
August was to have been a celebration for Stewart, the first anniversary of the dirt track wreck that changed his life. He tweeted this message to fans a week ago: "1yr ago today my life changed. Thank you to everyone that worked so hard to get me back to where I'm at today. It's your life, live it!"