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David Perron's second goal of the night put the Blues ahead with just over five minutes to play and Ryan O'Reilly scored into an empty net to finish a four-point game as the Blues beat the Senators 6-4 on Thursday night in Ottawa.

The Blues trailed 2-0 and 3-2 and came back to win their third in a row and improve to 3-0-1 on the young season. It was the second time this season the Blues won after trailing going into the third period.

"I thought our guys worked hard," Blues coach Craig Berube said. "They had to. They kept with it, kept battling, and got two points out of it."

Jake Allen stopped 25 of 29 shots to get the win, including three big saves on a 2-on-0 break. It was Allen's 137th win for the Blues, tying him with Curtis Joseph for second on the Blues' all-time list. 

"Those saves) could have been a turning point for sure," Berube said. "Big save, timely saves win you hockey games."

The Blues now head to Montreal for a game on Saturday.

Brayden Schenn had put the Blues ahead 4-3 with 7:13 to go in the game, his third game in a row with a goal, as he jumped on a loose puck in the Ottawa zone and scored. But 85 seconds later, Ottawa tied the game when the Senators' Anthony Duclair was left alone in the slot and scored to make it 4-4.

But that lasted only 42 seconds. Goalie Jake Allen came out to play a long Ottawa pass and sent it back up ice to O'Reilly, who flipped it on Tyler Bozak, who passed to Perron, whose shot hit the top of goalie Anders Nilsson's pad and when in under the bar.

"He's just a competitor," Berube said. "If you watch him closely, he competes on every puck all the time. That's what makes him a really good player. He's got a great shot. He used it tonight and got us a couple."

Ottawa pulled Nilsson with about 2 1/2 minutes to go and after a minute in the Blues end, the Blues won a faceoff and Bozak fed O'Reilly for the empty netter. It was the second career four-point game for O'Reilly, who assisted on the Blues' first, third and fifth goals.

The Blues went into the third period down 3-2 but got even just 1:38 in on a goal by Jay Bouwmeester on a shot from the point with help from a screen from Sammy Blais in front of Nilsson.

The Blues battled back from a two-goal deficit to get even with Ottawa in the second period, and then gave up a shorthanded goal to trail 3-2 after two periods.

It was the second shorthanded goal allowed by the Blues this season in just four games (second only to San Jose's three), and matched the number of power-play goals they have. Chris Tierney grabbed the puck after David Perron and Justin Faulk didn't hook up on a pass attempt back to the point and Tierney held off Alex Pietrangelo long enough to score at the near post, 10 seconds into the power play.

The Blues got goals from Perron and Oskar Sundqvist to get even in the second after a lackluster first period.

After seven straight failed power plays, Perron scored. O'Reilly rimmed the puck behind the net to Perron on the right side, who skated in, did a little toe drag, and fired past Nilsson for the 200th goal of Perron's career.

The goal came less than a minute after Jake Allen made three saves on a two-on-none break for Ottawa (Robert Bortuzzo fell on the play). Shortly before, Allen made a save on a three-on-one break.

Two and a half minutes later, Ivan Barbashev won a draw, Sundqvist skated the puck out of the circle and put in a slapshot for a goal in his second game in a row. 

The game's first intermission came a little earlier than usual. The referees signaled the period over and sent the teams to their dressing rooms with 2:10 to go in the period after a pane of glass broke. The time was tacked on to the second period.

Both teams had just eight shots on goal in the period, but the Blues' shots were much slower in coming. The Blues had a power play after Bobby Ryan went off for tripping and managed just one shot on goal.

Ottawa, winless in its first four games and expected to be the worst team in the league this season, scored first on a goal by Vitaly Abramov 8:10 into the first period. Jake Allen, getting his first start of the season in goal, went behind the net to clear the puck but it hopped over this stick and the Senators got the puck to the net before Allen could get back.

The Senators got their second goal got their second goal in the finishing section of the period, with the puck taking a bounce out to Artem Anisimov in the slot, who beat Allen.