The Carolina Panthers have not had a very good week: Running back Stephen Davis was lost Friday with a knee injury, just days after star receiver Steve Smith broke his leg.

Davis and Smith are the defending NFC champions’ top two offensive threats.

Davis, who ran for a career-best 1,444 yards last season, began having problems with his knee after practice Thursday, and an MRI showed “a small cartilage problem,” coach John Fox said Friday.

He had arthroscopic surgery Friday, and will miss Sunday’s game at Kansas City. The Panthers said he would be out two to five weeks.

Smith, who led Carolina in receptions (88) and yards receiving (1,110), is out indefinitely after breaking his left leg in Monday night’s loss to Green Bay.

DeShaun Foster will replace Davis at running back, and rookie Keary Colbert is expected to start at receiver. Chris Gamble, another rookie, could replace Smith on punt returns.

“As I told the team, we have got 53 men out here we think we can start and win for us. They have got to step up,” Fox said. “It is part of the game. Kansas City is going to play without some starters. These guys understand that. It is not the first time it has happened.”

Foster was a solid backup to Davis last year, and started two games in place of him while Davis nursed an ankle injury. He ran for 429 yards on 113 carries, and had 26 receptions for 207 yards and two touchdowns.

Foster was also solid in the postseason, running for 196 yards and two touchdowns in four games.

He first proved his worth in a regular-season game at Indianapolis, filling in for Davis when he left in the second half with a bruised arm. Foster came in and carried 16 times for 85 yards to help the Panthers to an overtime victory.

Serena Williams, right, argues with chair umpire Mariana Alves, left, during her match with Jennifer Capriati at the U.S. Open in New York, Tuesday, Sept. 7, 2004. Williams came up on the losing end when Alves overruled the line judge in the opening game of the third set. Capriati went on to win 2-6, 6-4, 6-4 and moved on to the semifinals.

With rain washing out live matches, people had a chance to see the officiating error that took a point from Williams and gave it to Capriati in the third set’s opening game Tuesday night.

That replay was everywhere. Well, everywhere, that is, but the chair umpire’s stand during the match itself.

And with no new results to talk about, the buzz around the National Tennis Center on Wednesday afternoon was about whether instant replay or other tools should be used to help make better rulings. There were at least two other questionable calls in the final game of Capriati’s 2-6, 6-4, 6-4 win.

“It was such a horrendous call. If I was closer to the court, I would have run onto the court,” said John McEnroe, who called the match on USA. “I’ve seen a lot of bad calls in my day, but I don’t think I’ve seen three bad calls in a set like that.”

Not surprisingly, given his “You cannot be serious!” reputation for on-court arguments, McEnroe favors adding some form of replay to tennis.

He would put limits, though, as there are in other pro sports, such as perhaps some sort of penalty for a wrong challenge. In the NFL, for example, a team can get up to three chances per game to question calls; if it loses a challenge, it forfeits a timeout. Limited forms of replay are used in the NHL and NBA, too, but baseball has never shown any interest in trying it.

Serena Williams demonstrated that her shaky knee is holding up fine, sweeping past Patty Schnyder 6-4, 6-2 yesterday to reach the quarterfinals of the U.S. Open.

Once again dressed in black, the two-time Open champion had little trouble in winning her fourth straight match at Flushing Meadows. Williams took this one with her serving, finishing with 12 aces compared with one double fault.

Williams was forced to pull out of last month’s Olympics because of pain in her left knee. She had surgery on it in August 2003, and missed 8-1/2 months while recovering.

In fact, she said a few doctors advised her to skip this tournament.

“My knee is doing pretty good right now. I haven’t had any worries,” she said. “I’m just excited to be here because I wasn’t supposed to come.”

Williams advanced to yet another matchup with Jennifer Capriati, who beat Ai Sugiyama 7-5, 6-2.

Williams is 10-6 lifetime against Capriati, including 3-3 in Grand Slam events. Capriati defeated her in the French Open quarterfinals this year, then lost to Williams in the Wimbledon quarters.

“I like playing Jennifer a lot, I really do,” Williams said. “We always play each other. We really know each other’s games.”

Of Williams, Capriati said, “I’ve played her at her best. I’ve played her at her worst. I’ve played her at my best, me at my worst. I can just only worry about myself, really.”

The criminal case against Kobe Bryant case collapsed Wednesday as prosecutors dropped the sexual assault charge against him, saying they had no choice because the NBA star’s accuser no longer wanted to participate.

Bryant, whose trial had been days from opening arguments, responded with an apology to the woman who had accused him and whose civil suit for damages is still pending.

“Although I truly believe this encounter between us was consensual, I recognize now that she did not and does not view this incident the same way I did,” Bryant said. “I now understand how she feels that she did not consent to this encounter.”

The woman’s attorney, John Clune, said she has been through an extremely difficult time since she alleged she was raped, and that she was disturbed by a series of courthouse mistakes that included release of her name and medical history. She has been the subject of death threats and relentless media coverage in the high-profile case.

“It is in her sincere belief that when this case ends, she does not want to be brought back into the criminal process,” Clune said.

“The difficulties that this case has imposed on this woman the past year are unimaginable.”

The dismissal marks a dramatic but not entirely unexpected turn in the high-profile case against one the NBA’s brightest young stars. For months, prosecutors had insisted they had a strong enough case to win a conviction beyond a reasonable doubt, but legal experts said their case was greatly weakened when Bryant’s attorneys convinced the judge to allow some evidence about his accuser’s sexual history.