CHICAGO - Chris Pronger had his uniform on straight, his helmet was positioned properly, and his skates were laced tightly.

Otherwise, his game was upside down, and the Flyers caught a strong dose of what happens when their premier defenseman, the player who was having the most significant impact in the Stanley Cup Finals, has a rough night.

They lose.

It doesn't take a hockey aficionado to realize the Flyers would be lounging at their summer cottages if No. 20 wasn't patrolling the blue line, using his mean edge, his stick and his vast experience to fend off trouble.

Certainly, Pronger is entitled to a bad game. He had one in Game 3 of the Eastern Conference Finals against Montreal, and the Habs got their only win of the series, 5-1.

But Pronger's timing couldn't have been much worse as his struggles helped lead the Flyers down the path of a 7-4 loss to the Blackhawks on Sunday at the United Center.

The win gave Chicago a three-games-to-two lead with Game 6 Wednesday at the Wachovia Center, and it backed the Flyers into a corner. They now have no margin for error if they are to win their first Cup since 1975.

Pronger, booed when he carried the puck, was on the ice for six Blackhawks goals, and he was in the penalty box when Chicago built the lead to 5-2 on a power-play goal by Dustin Byfuglien late in the second period.

Byfuglien is the 257-pound power forward who entered the series with eight playoff goals. Pronger had made sure he was stuck at eight, refusing to yield the front of the net to the bullish Byfuglien, relentlessly unnerving him.

So Byfuglien must have felt as if he was sprung from a trap as Pronger sat in the penalty box.

Just about everything went wrong for Pronger, and much of it was his own doing. On the Blackhawks' first goal, Pronger closed his legs in an attempt to block Brent Seabrook's shot. But the shot ticked off him and trickled past goalie Michael Leighton for Chicago's second power-play goal of the series.

After Seabrook scored, Dave Bolland and Kris Versteeg were in Pronger's vicinity, yet still had enough time and space to increase the Blackhawks' lead to 3-0.

Pronger's grim night ended when Byfuglien beat him to the puck and sent it into an empty net.

Pronger's influence on this series had been so forceful that it led Blackhawks coach Joel Quenneville to break up his top line of Jonathan Toews, Patrick Kane and Byfuglien, a trio that had combined for 22 goals and 34 assists before the Finals. That line had one goal and four assists in the first four games.

Quenneville put Toews between Tomas Kopecky and Marian Hossa; Kane alongside Patrick Sharp and Andrew Ladd; and Byfuglien with Bolland and Versteeg. Kane and Byfuglien had goals.

Before the game, the Blackhawks begrudgingly acknowledged they had to do something to shake free of Pronger.

"We want to make him turn and make him play defense and make him play from the back end," Quenneville said. "He's what he is. We have to be more physical, and we have to make it harder on him and make it tougher."

Pronger has averaged more than 30 minutes of ice time a game during the Finals, with no sign of wearing down. The Blackhawks were more intent on banging him around in Game 5, and Pronger absorbed big-time hits, most notably by Byfuglien, who drilled him into the boards in a corner behind the Flyers' net and sent him sprawling to the ice in the second period.

The good news for the Flyers is that Pronger gets two days of rest before Game 6, and that he has yet to play poorly two games in a row.