Having entered the fourth hour with its cars holding the top two spots at Daytona, Chip Ganassi Racing found life a little more difficult as the race reached one-quarter distance.

Although the #01 Riley-Lexus of Scott Pruett, Salvador Duran and Juan Montoya led through the end of the sixth hour, it had not had things all its own way during the three-hour period, and had also lost the company of its #02 sister car, which suffered a brake fire on pit-road and then lost more time with a slow lap back to the pits in the sixth hour after a tyre blew and damaged the bodywork.

Rolex rookie Montoya consolidated Ganassi's lead in the fourth hour, although the #91 Lowe's Riley-Matthews entry, piloted for the most part by the Colombian's NASCAR rival Jimmie Johnson, continued to apply pressure to the leader, with the second Ganassi car, of 2006 race winners Dan Wheldon and Scott Dixon - teamed this year with Memo Rojas - completing the top three.

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While pre-race favourites SunTrust Racing, MSR and GAINSCO continued with their respective comeback efforts, the #39 Cheever Racing moved into fourth spot, ahead of the #77 Doran and #60 MSR cars. Also in the mix remained the #58 Brumos and #05 Sigalsport entries, both putting pressure on the leaders between driver changes.

There were problems at the end of the hour for Katherine Legge, who wiped the nose from her #84 Robinson Racing Riley, and the #58, with Darren Law at the wheel, which was penalised for leaving the pit-lane complete with the fuel hose still attached. Somehow, the ensuing stop-go penalty only dropped the Brumos car a lap off the lead and, with problems about to strike those at the front of the field, the Red Bull machine remained in contention.

The #91 Riley-Matthews entry was also in the wars, having been collected by an errant rival, but also managed to stay in touch despite a prolonged stop for repairs to rear-end damage. There was less good news for the #75 title-winning Krohn team car, however, which pitted with ECU problems - not the first to report such trouble.

The #01 car maintained its position at the head of the field through the fifth hour, with Duran taking over from Montoya and gradually dropping rivals from the lead lap. Remarkably, it was the #58 Brumos entry of Law, Scott Sharp, Buddy Rice and David Donohue that continued to apply the greatest pressure, recovering to the lead lap, while the #02 and #91 dropped back.

The promising run being enjoyed by Eddie Cheever's team also faltered during the hour, allowing the #11 SAMAX/CITGO entry crewed by Milka Duno, Patrick Carpentier, Darren Manning and Ryan Dalziel to inch up the order, moving ahead of the Riley-Matthews and works Doran cars, and the #59 Brumos car to join its sister in the top five. Krohn's lead car also continued to struggle against gremlins, but its #76 stablemate continued to keep the outfit's vivid green-and-blue colours in the top ten, where it was joined by the underdog AIM Autosport squad.

The #58 inherited the lead during hour six, when Ganassi made a precautionary brake pad change during a scheduled pit-stop, but Buddy Rice found himself quickly being hunted down by Scott Pruett. As the sixth hour ended, the red-and-white machine just managed to sneak back into top spot, with the Red Bull car pit-bound for a driver change.

The #11 SAMAX machine had moved up to third spot, ahead of Ryan Hunter-Reay in the #91 and Joao Barbosa, who held the second Brumos entry in fifth. The Portuguese ace was under pressure from Bill Auberlen in the ever-present #05 Sigalsport entry, but both cars ran a couple of laps off the lead, along with the #77 Doran.

While the #02 dropped to ninth on the road after its problems, the #10 SunTrust car broke back into the top ten during the sixth hour. Likewise, the Dyson/Howard Motorsports car piloted by Chris and Rob Dyson, along with Brits Olly Gavin and Guy Smith, was back up to 13th after losing time with problems of its own. Legge's panel-damaging incident did not keep the #84 car down for long, and Paul Dallenbach ended the sixth hour in 16th spot, about where it had run for the duration.

The #60 MSR car had dropped to 28th overall, but that was still a long way ahead of early victims Howard Motorsports, who #20 had climbed back to 34th, and Vision Racing, which, with Tony George at the wheel, was running 38th. Further back still, Krohn's #75 was down in 51st, one behind the #6 MSR entry with Paul Tracy and AJ Allmendinger among the driving strength, and four ahead of the luckless #99 GAINSCO crew, which continued to press on after its lap four incident, despite now being 51 laps shy of the leader's 197.

The GT class continues to be a battle between the #74 Tafel Racing and #85 Farnbacher Loles Porsches, which both had 183 laps under their belts, one more than the third in class #73 Tafel entry.

Leading positions - Rolex 24: 6hrs

1. Pruett/Montoya/Duran Ganassi 197 laps
2. Law/Sharp/Riuce/Donohue Brumos 197
3. Carpentier/Duno/Manning/Dalziel SAMAX 196
4. Goossens/Matthews/Johjnson/Hunter-Reay Riley-Matthews 196
5. Haywood/FRance/Moreno/Barbosa Brumos 195
6. Alhadeff/Auberlen/Sigal/Wendlinger Sigalsport 195
7. Gollin/Gidley/Servia/Jourdain Doran 195
8. Jonsson/Krohn/Said Krohn 194
9. Frisselle/Frisselle/Wilkins/Empringham AIM 194
10. Dixon/Wheldon/Rojas Ganassi 194

GT

1. (22) Farnbacher/Henzler/Lux/Tafel Tafel 183 laps
2. (23) Ehret/Keen/Wener/Hardt Farnbacher Loles 183
3. (24) Swartzbaugh/Davis/Liddell/Muller Tafel 182
4. (25) Lacey/Mowlem/Wilkins/Papadopoulos/Arnold Doncaster 182
5. (26) Westbrook/Johnson/Huisman/Lietz Synergy 181