In two seasons of coaching the Philadelphia Flyers to remarkable hockey success, Mike Keenan has won a reputation as the consummate modern hard guy, a no-nonsense, nose-to-the-computer-terminal, chest-out, chin-up, dawn-to-dusk overachiever.

In fact, however, Keenan is one of the original Nice Guys.

"It's true," he admitted sheepishly in his spartan office at the Flyers' training rink. Nik and the Nice Guys was a band at St. Lawrence University in upstate New York, from which Keenan graduated a decade and a half ago. He sang, even took lead on a few numbers. He won't say which.

"It was a group of athletes that are professionals now -- doctors, lawyers," said Keenan. "The amazing thing is, the band is still together."

Except for him.

No more Mr. Nice Guy.

The transformation is complete, his players are quick to agree.

"He's always hard on everybody," said center Peter Zezel. "He doesn't ease up. He's upset when we lose, but he's hard even when we win."

"He can make life unbearable," said Bob Froese, the Flyers' No. 1 goalie since Pelle Lindbergh's tragic death in a car crash in November. "You hate him. He won't talk to you; he doesn't even acknowledge you. You feel like two cents.

"You shake your head at the time, but then later, you say, 'There was a master plan,' " added Froese, who in four months went from expendable backup to a regular with the best goals-against average in the National Hockey League. "It's a different way of handling things, but it seems to work. He's won everywhere he's been. You can't argue with success. All anyone on this team wants to do is win."

Win the Flyers do under their uncompromising, 36-year-old leader. When they beat Pittsburgh Thursday night, 5-1, in front of the usual horde of 17,000-plus screaming partisans at the Spectrum, it was Keenan's 100th victory. That makes him only the second NHL coach to win 100 in his first two seasons, the other being Tom Johnson, who won 111 with Boston in 1970-72. The difference is, Johnson inherited a Stanley Cup champion. Out of Playoffs

"When Keenan came, we'd been knocked out of the playoffs in the first round three straight years," said Flyers President Jay Snider. "We had the youngest team in the league. We were picked to finish fourth in our division. Instead, we made it to the Stanley Cup finals losing, 4-1, to Edmonton . It was an incredible accomplishment. It was enormous."

Many critics expected Keenan to fall prey to a sophomore jinx this season, the victim of his own tyrannical aloofness from his players. But the Flyers, whom he will lead into Capital Centre today to face archrival Washington, are right at the top of the Patrick Division pile, three points ahead of the Capitals after yesterday's 4-2 victory over the New York Rangers.

So who needs a Nice Guy?

Snider believes Keenan, who came to Philadelphia after leading the University of Toronto to the Canadian collegiate title in 1984, "brought coaching into the 1990s" by relying on computerized information and videotapes of opponents to prepare for games, and by being a professional rather than an ex-player taking up a new trade. "Coaching is his career. It's not accidental," said Snider.

Nor is it accidental that Keenan keeps his distance from players. After the Penguins game, he popped into a news conference just long enough to downplay the 100th win, then dashed to his private office across the hall from the dressing room and slammed the door with a clang. He was not seen again.

On Friday at practice here, he ran drills with Prussian efficiency, then vanished into his office. The shades were drawn, the door shut and nothing about the circumstances invited intrusion.

"When other coaches are drinking beer with the boys," said Snider, "he's looking at films."

Keenan's on-ice captain is center Dave Poulin, a jut-jawed, straight-shooting Notre Dame graduate, a Steve Garvey on skates. He's one of the people Keenan refers to when he talks about "solid citizens" who play for him.

Poulin says Keenan's favorite management tool is unpredictability. "He works at it," said Poulin. "Take December of last season . On the 23rd, we beat Washington here. Everybody figures, easy practice on Christmas Eve. So he gives us the hardest practice of the entire two years. We skated until we dropped. Then we lose at Washington the day after Christmas.

"But after that," said Poulin, "we went on a tear. Now, I know he wasn't looking for that loss. But what about the tear?"

Keenan admits he likes to keep his players off balance by doing the unexpected. Interest High

"It's part of keeping the interest of your club high," he said, "and of creativity. You try to create challenging situations for them individually and collectively. That way they stay out of the grooves and ruts.

"Individually, you ask them to do things they might not think they're good at. It's uncomfortable for some people. There's a lot of anxiety, but anxiety isn't the creative tool; it's seizing the opportunity to do something you're nervous about. Try it. So what if you fall on your face? No harm in that. Many of us do. It's getting up and trying again that breeds success."

And no one is immune. Even the Flyers' top scorer, Tim Kerr, who has 50 goals for the third straight season, finds himself increasingly relegated to a role-player as designated scorer during power plays. Most of his ice time these days comes on power plays. 'No-Star System'

"You know, it's a no-star system here, which I think is good," said Kerr, unconvincingly.

If you had to put a name on the Flyers' style under Keenan, "relentless" would be as good as any.

"They just keep coming at you," said 37-year-old goalie Chico Resch, who joined the club two weeks ago to back up Froese down the stretch and into the playoffs. "That's the most impressive thing about the Flyers, and that's how Mike works.

"This is the hardest and shortest I've ever practiced in my life," Resch said as he sat dripping after a 40-minute workout. "Most teams go half-again this long, but Mike's philosophy is quality, not quantity. It's the most efficient practice I've ever seen. If you take a water break here, it's 30 seconds, then back to work."

Snider sees considerable similarities between Keenan's operation and Bryan Murray's at Washington. Both teams stress defense, conservativism and mistake-free performance. "You're looking at the two finest programs in professional hockey," he said.

Yet rumors persist of bad blood between Murray and Keenan. Keenan denies it. "We're just two guys that thrive on competition," he said. "There's nothing wrong with wanting to win."

Spoken like a true former Nice Guy.