Seguin, who entered the game without a point in the series, veered slightly to his right to draw goaltender Braden Holtby from the net, then lifted a shot past the rookie to win it.
That kept alive Boston's bid to become the first team to win successive Stanley Cups since the Detroit Red Wings in 1997-98.
Thomas had 35 saves in regulation, but it wasn't enough to prevent the third overtime game of the series. All six games have been decided by one goal.
So close has the series between the Phoenix Coyotes and Chicago Blackhawks been that one little bounce of the puck, one fumble along the boards, or one faceoff win can be the difference between celebrating a victory and swallowing a loss.
Five straight games have been decided in overtime, and who would dare say it won't go to six when the teams meet again Monday night with the Coyotes leading by three games to two.
It's so even and unpredictable that both teams have triumphed twice on the road, with Chicago's 2-1 victory Saturday night in Glendale, Ariz., sending the series back to the United Center.
"We're going home, which is going to be good, and try to get a lift from the fans and try to get a big win," Chicago defenseman Brent Seabrook said after Game 5.
The series is tight and testy. Each team has had a player suspended. Chicago's Andrew Shaw got three games for running into Phoenix goalie Mike Smith in Game 2. And Coyotes forward Raffi Torres was slapped with 25 games for his shoulder-to-head blast on Marian Hossa in Game 3.
- Associated Press