When the Celtics and Philadelphia 76ers met during the regular season, the objective was first place in the Atlantic Division, once considered an innocuous title, a goal not worth mentioning for teams with greater expectations. But there was incentive to take the division, since it assured the winner of no worse than a No. 4 playoff seed, which meant avoiding Chicago and Miami in the first round. All that sweat, toil, and injury risk, it turned out, might not have been worth the worry. Home-court advantage and seedings were rendered irrelevant as the Celtics and No. 8 seed Sixers have reached the Eastern Conference semifinals, which start Saturday night.The Celtics’ emphasis on health over home court paid off in a 4-2 elimination of the Atlanta Hawks. And the Sixers shocked the top-seeded but injury-reduced Bulls. Expect this series to be an old-style defensive wrestling match. The Celtics led the league in defensive field goal percentage (.419) and the Sixers were third (.427). The Celtics held the Hawks to 40.1 percent shooting in the series and 80 or fewer points in three games. The Sixers clanked their way to 40.7 percent shooting against the Bulls and held Chicago to 42.1 percent, three games finishing with both teams scoring fewer than 80 points. And Celtics coach Doc Rivers is thinking of even lower numbers. “What Philly did was, they defended Chicago,’’ Rivers said before Friday’s practice. “Chicago really struggled scoring. We talk about Philly and their athleticism and their running, but what people keep forgetting is they’re not a good defensive team - they’re a great defensive team. “They’re going to try to make us struggle scoring and we’re going to make them struggle scoring, too.’’ Rivers sees beauty in low-scoring games. “If the game’s 50-50 and close and competitive, I don’t know why that’s ugly,’’ he said. “I’ve always argued against that. I guess 120-121 is more exciting. I think being competitive is more exciting.’’ But Rivers is understandably hoping to avoid the wide-open conditions that characterized the Celtics’ losses in Philadelphia in March - 103-71 and 99-86. But the Celtics adjusted, taking a 103-79 home win over the Sixers April 8. “I will say I’m happy they beat us in the fashion that they did during the regular season,’’ Celtics guard Ray Allen said, “because beating them, 3-0, if we did during the regular season, you come in a little too cocky and arrogant. But since they beat us two times out of three, we know this team is capable - they’ve had our number. So, we have to be focused on taking care of all those things we didn’t do well.’’ During that April loss, Philadelphia coach Doug Collins expressed frustration in finding ways to motivate the Sixers without damaging their confidence. But Collins has apparently succeeded. “[Collins] has done a good job giving them their identity,’’ Rivers said. “They’ve figured out that if they can defend and get multiple stops and turn the ball over, then they can run. The other team is making the shots, then they can’t run. So, they know exactly who they are and who they want to be. “When they get into transition, they score. When they get into the halfcourt, then it’s a little more difficult. And that’s going to be our test. In the three games we played, in one game we did a better\"