Kessel added an assist, while Lupul had two of his own.
Daniel Alfredsson scored twice for the Senators (0-2-0). Colin Greening, Jason Spezza and Stephane Da Costa, with his first in the NHL, had the other goals for Ottawa. Erik Karlsson chipped in with three assists.
Alex Auld took the loss in relief for Ottawa after allowing two goals on 11 shots in relief in the third period. Craig Anderson started and gave up four goals on 20 shots through two periods.
``We played good for 50 minutes and took our foot off the pedal,'' said Maple Leafs' captain Dion Phaneuf, who has three points in two games. ``We got to learn from that, but it's a big two points for our team and we'll take them.''
After Greening and Orr exchanged goals to start the third period, Alfredsson made it a 5-2 game at 11:04 with a short-handed breakaway goal after taking a clearing attempt coming out of the penalty box.
The coach on Phil Kessel eating up seconds defensively in the Ottawa zone late in the game
"He did an outstanding job. I put him out there and probably everybody questioned my sanity but I rewarded him for his three goals and he worked hard all night. He showed a lot of determination in that situation. So it was great for Phil, great for his confidence." -- Ron Wilson
Then the Leafs gave the Senators a 5-on-3 man advantage and Ottawa struck with a pair of power-play goals eight seconds apart. Spezza scored at 15:36 and Alfredsson got his second of the game on a neat deflection.
Kessel completed his hat trick at 17:41, snapping a shot over Auld's glove hand on a 2-on-1 rush, and the Leafs looked to have quelched the Senators' comeback attempt.
But Ottawa struck back when Da Costa took a feed from Karlsson and scored at 18:06 to set up a wild end to the period.
The Senators pulled Auld for an extra attacker but couldn't quite complete the comeback as the Air Canada Centre crowd breathed a huge sigh of relief at the final buzzer.
The Leafs' letdown overshadowed a four-point night by Kessel, who continues to evolve into a more complete player.
``He played really well the last two months of last season and he's continuing where he left off,'' said head coach Ron Wilson. ``We've asked him to be a little more aggressive on the forecheck and he was doing that.
``Tonight he was getting in there really quick.''
Wilson rewarded Kessel for his hard work by putting the winger out to protect the 6-5 lead in the last minute of the game.
According to Phaneuf, the Leafs' star sniper has been Toronto's top player up front so far for the team.
``Phil has been extremely good for us both games,'' Phaneuf said. ``He's been one of our best forwards, if not our best.
``And not just scoring goals. He's been strong in both ends of the ice for us.''
Ottawa came out strong to open the game, but it was Toronto that took a 2-0 lead into the first intermission on goals by Grabovski and Kessel.
The intensity between the provincial rivals picked up in the second period.
Toronto took a 4-0 lead into the second intermission when Kessel scored his second of the game after taking a nifty pass from Tyler Bozak and deking around Anderson.
The Leafs' top line of Lupul, Bozak and Kessel finished with eight points.
Toronto went 2-for-9 with the man advantage, while Ottawa wound up 2-for-8.
Notes: The Senators lost their season opener 5-3 in Detroit on Friday. ... Orr took Jay Rosehill's spot in the lineup for Toronto. ... Ottawa plays back-to-back games 16 times this season. ... The Maple Leafs started the season tied for the second youngest roster (26.3) in the league behind the Nashville Predators. ... The Leafs have a week off until they play again when the Calgary Flames come to town next Saturday. The Senators return to Ottawa for their home opener Tuesday against the Minnesota Wild. ... Announced attendance was 19,324.
Saturday, October 15
vs. Calgary Flames 7 PM ET (CBC, AM640)
The Maple Leafs will continue their five-game homestand as they have a full week off before welcoming the Calgary Flames to town. Starting on Monday, the Leafs will be in Trenton, Ontario for three days of bonding and practice.
When the Flames are here next Saturday, it will be the final game of their three-game road trip. They make stops in St. Louis and Montreal prior to playing the Leafs.