Denver led 23-10 against the Giants when Jason Elam hit a field goal with 13:18 left, but the Broncos gave up two touchdowns in the final 9:07, including a 2-yard touchdown pass from Eli Manning to Amani Toomer in the closing seconds of a 24-23 loss.
The Broncos held on to win the previous two weeks despite fourth quarter rallies by New England and Washington.
"You've seen it coming the past couple weeks," tight end Stephen Alexander said. "We've let teams get back in the game and we've held on to win a couple, but today we couldn't do it."
The Broncos didn't appear to make any major schematic changes in the fourth quarter, but they had some costly mistakes, including a missed field goal by Elam that would have given Denver a late nine-point lead.
The Broncos are still in first place in the AFC West at 5-2, but they blew a great opportunity to stay 1 1/2 games up on Kansas City for the division lead.
"Against Washington and New England, we got off pretty lucky because there wasn't enough time left," defensive end Ebenezer Ekuban said. "This week it finally caught up to us."
Broncos coach Mike Shanahan called the loss to the Giants one of the toughest of his career.
"It's up there," Shanahan said. "It's always tough when you feel like you have the game under control and you have a few opportunities with field position and you don't take advantage of it."
Eli Manning's 2-yard touchdown pass with five seconds remaining completed a fourth-quarter comeback for the Giants as they beat the Broncos 24-23.
Less than two minutes into the fourth quarter, the Broncos took a 23-10 lead, but they allowed the final 14 points of the game. Denver's defense allowed 163 yards through three quarters but 148 yards in the fourth quarter.
The Broncos got 180 rushing yards from their tandem of Mike Anderson and Tatum Bell, and Jake Plummer didn't throw an interception for the fifth straight game, but the Broncos couldn't sustain a drive late in the game to run out the clock.
--WR Darius Watts, a second-round pick last year and Denver's third receiver as a rookie last season, was inactive for the first time in his career on Sunday. He has slid down the depth chart and rarely has played this season.
--RB Ron Dayne's homecoming to New York never materialized. He was inactive for the second time this season against the Giants on Sunday. Dayne spent his first five seasons with the Giants.
-- RB Mike Anderson had his second 100-yard rushing game of the season when he ran for 120 yards and a touchdown against the Giants. Anderson is a tough runner, and he is still the starter ahead of Tatum Bell.
--CB Champ Bailey gave up a touchdown to Giants WR Plaxico Burress, but he also had an interception. Bailey didn't have any problems with his injured hamstring. He hadn't started and finished a game since Week 2.
--RB Tatum Bell only got eight carries against the Giants but had 60 yards. He broke another long run, gaining 37 yards on a toss play in the second quarter.
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