The Senators scored twice in the third period to come back from a 3-1 deficit.
Greening was in the corner and found Condra alone in the slot and he beat Reimer high glove side at 2:23 to make the score 3-2.
Karlsson tied the game on a power play at 10:30 of the third after sneaking in from the point and scoring from the top of the crease following a pass from Mika Zibanejad
Senators forward Zack Smith would just as soon forget everything related to the Leafs' second goal, starting when he was knocked down just inside the Toronto blueline and turned the puck over.
Van Riemsdyk was the initial recipient of the turnover as he gained control of the loose puck and started up ice on a 2-on-1 that was eventually turned into a goal by Kessel at 3:05 of the second period.
The bad news didn't end there for Smith. Following the goal, thinking there should have been a penalty on the play, he was given a two-minute unsportsmanlike conduct penalty along with a 10-minute misconduct.
The Leafs then scored another semi-controversial goal at 11:37 of the second period as Gardiner scored his first goal of the season with Van Riemsdyk lying in the Senators' net partially on top of Anderson.
Problem was Senators defenceman Marc Methot was the one who shoved Van Riemsdyk into the net.
The teams had emerged from the first period tied 1-1 after both teams capitalized on early power plays.
In the middle of traffic in front of Anderson, Van Riemsdyk was able to get his stick on a loose puck and put a backhand shot through the goaltender's legs at 1:37.
A little more than eight minutes later MacArthur drew the Senators even with a one-timer off a pass from Zibanejad through the legs of Reimer.
Notes: The game was the second of four meetings between the teams this season. Toronto won the first meeting 5-4 in a shootout on Oct. 5 at Toronto's Air Canada Centre…Senators forwards Bobby Ryan and Milan Michalek, who turned 29 Saturday, haven't scored in eight games,,,Forward Matt Kassian was in the lineup for the first time in the Senators' last 11 games…The Maple Leafs have the third youngest roster in the NHL with an average age of 26.8 years…Paul Ranger and Morgan Rielly were scratches for the Leafs while Eric Gryba and Cory Conacher were scratches for the Senators…The leafs have lost 80 man games to injury this season compared to just 15 for the Senators.