Reigning AMA Supercross champion Ricky Carmichael has halted James Stewart's perfect start to the 2007 season by winning a thrilling round four at San Francisco's AT&T Park.

Carmichael, riding only a partial 2007 season as he winds down his record-breaking career, was second to Stewart in the first two rounds - then sat out round three - but came back with a bang on Saturday night.

Yamaha's Chad Reed grabbed the $1,500 Progressive Direct Holeshot Award to start the 20-lap main event in front, but the 'big three' were soon fighting for victory as the Australian was quickly caught by Stewart and Carmichael.

The opening laps saw no less than five lead changes as each took turns in front, but two crashes by Stewart and another by Reed ultimately cost them a shot at the win while Carmichael remained solid for a hard-fought victory.

"These guys were riding buck wild out there tonight," said Carmichael in reference to the crashes by Stewart and Reed. "The track was pretty rutty and I was just laying back and taking advantage of what was happening in front of me. I'm happy to win again in San Francisco and the next time you'll see me is at Atlanta."

Reigning world champion Stewart eventually brought his Kawasaki home in second place to extend his points lead over third placed Reed in both the AMA and World Championship standings.

In the Supercross Lites class, Kawasaki's Chris Gosselaar earned the Holeshot Award but was quickly dropped to the number two position by his championship leading Kawasaki team-mate Ryan Villopoto. Once out front, Villopoto began to pull away and was later aided by a crash that took out Gosselaar and KTM's Martin Davalos, who was running in third.

Villopoto took the win, his third of the season, while Honda's Josh Grant and Jake Weimer finished second and third, respectively.

Amp'd Mobile Supercross continues next Saturday, February 3, for the last of three races at Anaheim's Angel Stadium.

Amp'd Mobile AMA Supercross Series Event Results, San Francisco:
1. Ricky Carmichael, Tallahassee, Fla., Suzuki
2. James Stewart, Haines City, Fla., Kawasaki
3. Chad Reed, Tampa, Fla., Yamaha
4. Timmy Ferry, Largo, Fla., Kawasaki
5. Nick Wey, Murrieta, Calif., Honda
6. David Vuillemin, Menifee, Calif., Honda
7. Travis Preston, Hesperia, Calif., Honda
8. Michael Byrne, Newnan, Ga., Suzuki
9. Jeff Gibson, Blacklick, Ohio, Kawasaki
10. Kevin Windham, Centerville, Miss., Honda
Amp'd Mobile AMA Supercross Series Season Standings:
1. James Stewart, Haines City, Fla., Kawasaki, 97
2. Chad Reed, Tampa, Fla., Yamaha, 80
3. Ricky Carmichael, Tallahassee, Fla., Suzuki, 69
4. Timmy Ferry, Largo, Fla., Kawasaki, 68
5. Nicholas Wey, Murrieta, Calif., Honda, 62
6. Michael Byrne, Newnan, Ga., Suzuki, 57
7. Kevin Windham, Centreville, Miss., Honda, 52
8. Travis Preston, Hesperia, Calif., Honda, 50
9. David Vuillemin, Menifee, Calif., Honda, 42
10. Heath Voss, Mico, Texas, Honda, 41

Amp'd Mobile World Supercross GP Season Standings:
1. James Stewart, Haines City, Fla., Kawasaki, 139
2. Chad Reed, Tampa, Fla., Yamaha, 127
3. Ricky Carmichael, Tallahassee, Fla., Suzuki, 116
4. Timmy Ferry, Largo, Fla., Kawasaki, 102
5. Nicholas Wey, Murrieta, Calif., Honda, 90
6. David Vuillemin, Menifee, Calif., Honda, 79
7. Travis Preston, Hesperia, Calif., Honda, 69
8. Heath Voss, Mico, Texas, Honda, 66
9. Michael Byrne, Newnan, Ga., Suzuki, 64
10. Jeff Gibson, Blacklick, Ohio, Kawasaki, 61

Amp'd Mobile Western Regional AMA Supercross Lites Event Results, San Francisco:
1. Ryan Villopoto, Poulsbo, Wash., Kawasaki
2. Josh Grant, Riverside, Calif., Honda
3. Jake Weimer, Rupert, Idaho, Honda
4. Josh Hill, Yoncalla, Ore., Yamaha
5. Jason Lawrence, Carlsbad, Calif., Yamaha
6. Kyle Partridge, Las Vegas, Nev., Honda
7. Steve Boniface, Corona, Calif., Kawasaki
8. Josh Hansen, Lake Elsinore, Calif., KTM
9. Michael LaPaglia, Murrieta, Calif., Yamaha
Amp'd Mobile Western Regional AMA Supercross Lites Season Standings:
1. Ryan Villopoto, Poulsbo, Wash., Kawasaki, 97
2. Jason Lawrence, Carlsbad, Calif., Yamaha, 71
3. Jake Weimer, Rupert, Idaho, Honda, 59
4. Steve Boniface, Corona, Calif., Kawasaki, 55
5. Matthew Lemoine, Pilot Point, Texas, Yamaha, 54
6. Christopher Gosselaar, Victorville, Calif., Kawasaki, 51
7. Christophe Pourcel, France, Kawasaki, 48
8. Troy Adams, Homosassa, Fla., Suzuki, 47
9. Josh Hansen, Lake Elsinore, Calif., KTM, 46
10. Josh Grant, Riverside, Calif., Honda, 42