Kenseth's Ford finished 1.344 seconds ahead of Rusty Wallace, who was attempting to break a 60-race winless streak. Four-time Winston Cup champion Jeff Gordon was third, followed by Mark Martin and Dale Earnhardt Jr.
"Our last pit stop was a big, key point in the race," Kenseth said of a two-tire stop on his last trip to the pits. "The other thing was when we ran out of gas, we didn't get a lap down and that was a big help, too. But the last pit stop, to get two tires, our car really worked well on two tires today instead of four for some reason.
"It was actually faster on two tires than on four, so when we got two that last time, I got clean air and got up front. I was able to pull away right away and that was a big advantage."
Generally, a car that has four fresh tires will run faster than a car with two new tires and two used tires. But in Kenseth's case, crew chief Robbie Reiser was able to make the gamble for track position and that got him the lead on his final stop on lap 260.
Kenseth's victory was worth $211,895. He averaged 113.857 miles per hour around the 1.0-mile oval.
In addition to his fifth win of the season, it was Kenseth's 19th top-10 finish. It also was the third consecutive victory at Phoenix for Roush Racing. Jeff Burton won here in 2000 and 2001.
Points leader Tony Stewart finished eighth after running as high as second. His car developed a handling problem midway through the race, but he still is firmly in control of his bid for the Winston Cup title.
Stewart leaves Phoenix with an 89-point lead over Martin. He needs to finish 22nd or better next week at the season-ending event in Miami to clinch the championship.
"We didn't get hurt too bad today," Stewart said. "I feel like Homestead owes me one, so I'm going back there to reassert my domination."
Wallace is hoping to keep his streak alive of at least one victory in the last 16 seasons.
"We're getting closer and closer and still only have one more race to go," he said. "I hope we can do it. We're going to take our best bullet (in Miami). If it's meant to be, it will happen. I'd hate to have that streak of 16 years with a victory quit."
Kurt Busch led twice for 117 laps -- the most of any driver in the race. Just as Stewart's car developed a handling problem late, so did Busch's Ford as he was in third place on lap 265 but finished sixth.
Ryan Newman started on the pole but quickly gave way to a dominating early run by Earnhardt, who led for 22 laps between laps 7-28 and for 83 laps between laps 35-118. But Earnhardt's car ran out of fuel before making a pit stop. That dropped him to sixth after the pit stop series on lap 125.
Rookie Scott Wimmer crashed on the front stretch after he was tapped by Robby Gordon's Chevrolet. Both Stewart and Earnhardt were able to dive low and avoid any contact with Wimmer's mangled Dodge.
Stewart started 17th but quickly worked his way into contention. He was second behind Busch on lap 229, but two laps later, his car began to fade. Jeff Gordon passed him for second and Stewart's car showed signs of a slightly damaged right front fender.
Stewart was able to fight the ill-handling car for the remainder of the race and salvage a top-10 finish, keeping control of his championship charge.
Earlier in the race, Kenseth ran out of fuel before making a pit stop, losing valuable track position. He was able to make that up at the end of the race.
"I wasn't sure at all my day was done," he said. "If you say your day is done and you give up, then I wouldn't be sitting there right now. It made it difficult to pull of the win, but that's the first time since I've been with Robbie Reiser in 1996 that he has ever run me out of fuel."
In the bigger picture, only Stewart and Martin have a mathematical chance of winning the 2002 Winston Cup championship. If Stewart finishes 22nd or better at Homestead, he clinches the title. He also can win the championship if he finishes 24th or better and leads one lap, or finishes 25th or better and leads the most laps.
Martin's team owner, Jack Roush, still is fuming over the deduction of 25 championship points from last week's race at North Carolina Speedway in Rockingham. Martin finished second in that race to Johnny Benson Jr., but in a postrace technical inspection, the right front springs did not meet the required 4 1/2 coils. Martin's springs were 4 3/8.
Roush has turned over the decision of an appeal to team president Geoff Smith.
"I am so upset, so out of my mind, that I can't organize a decent thought about that," Roush said. "That's the third time something like that happened to me, and I don't forget about that."
If Martin had not been penalized, Stewart's lead after this race would be 64 points.
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UPI Sports Results for September 19, 2014