Guenther scored again at 16:59, sliding the puck between the legs of Thompson off a pass from Keller to give the Mammoth a 2-1 lead.
Logan Cooley scored a power-play goal at 19:53, tapping in a loose puck that was behind Thompson off a shot from Sergachev to make it 3-1.
“I thought the first period, we were moving it well,” said Cooley. “I think we kind of just started to let it slip, give up some odd-man rushes, and they capitalized. Every game is so important right now, and it stings.”
Ovechkin scored his first of the game at 5:01 of the second period, tapping in a shot by Sandin to cut the lead to 3-2.
“It's a hard situation for us, but you never know what's going to happen,” said Ovechkin.
Ovechkin then tied it 3-3 at 11:28 with a wrist shot from the point past Vanecek.
Beauvillier scored a power-play goal at 2:43 of the third period, tipping in a shot from Cole Hutson to give the Capitals a 4-3 lead.
“I think everybody was emotionally involved. It was a tough game on both sides,” said Beauvillier. “I think winning a game like that with lots of emotion feels great. We're still in the fight where we got two points, and we're going to fight till the end.”
Sandin extended the lead to 5-3 at 6:02, putting the puck on net from the point while Vanecek was out of position.
“Everyone needs to contribute with whatever they can,” said Sandin. “If it's scoring a goal or if it's blocking a shot, I think we did that tonight.”
MacKenzie Weegar made it 5-4 at 8:06, scoring with a wrist shot from the point past Thompson. It was his first goal with the Mammoth since being acquired from the Calgary Flames on March 4.
Miroshnichenko scored his second goal at 9:54, flicking a backhand through the legs of Vanecek to push the lead back to 6-4.