The Vikings offense has converted one second-half third down in two games.

One.

It came at the 9-minute, 58-second mark of the third quarter of the season-opening loss at San Diego when quarterback Donovan McNabb scrambled for 10 yards on third-and-7. Since then, the Vikings have gone 0-for-8 on second-half third downs.

That's an 11.1 percent success rate on nine third-down situations averaging 10 yards. That also explains why a run-oriented, ball-control offense has scored just three second-half points.

Meanwhile, not to be outdone in second-half frustration during this 0-2 start, the Vikings defense has allowed eight third-down conversions in the second half. Throw out the Chargers' kneel-down to end the game, and opponents have converted eight of 11 second-half third downs.

That's a 72.7 percent lack of success rate for the Vikings. That also explains why a defense that doesn't force a lot of turnovers has been bent and broken for 41 second-half points in losses that have included blown halftime leads of 17-7 and 17-0.

"We had some situations in the second half of both games where you can either make a first down or deny a first down that could have turned the game in our favor," coach Leslie Frazier said. "It's something that we have to get better at."

Offensively, McNabb is 0-for-5 passing and has been sacked twice on second- half third downs. Of course, it didn't help that penalties turned one third-and-3 into third-and-8 and another third-and-3 into third-and-18.

Critical mistakes such as those have created a sense among Vikings players that the Chargers and Buccaneers are less responsible for the team's 0-2 start than the Vikings are themselves.

"I think we all just sat in there [watching the Buccaneers game film] and it kind of made us mad knowing that they did nothing to beat us," receiver Percy Harvin said. "They didn't throw any looks at us that were confusing. We just had a letdown."

Harvin referred to a couple of third-down plays to illustrate the reasons for the team's frustration.

"We've just got to execute the play that was called a little better," Harvin said. "Twice, we were lined up in the wrong formation, so that's just little mistakes that we're killing ourselves. [If] we're in the right formation, we probably had two walk-in touchdowns."

Harvin presumably was talking about third-down plays in the red zone. But he wouldn't elaborate.

"I'm not getting into the formations and who did what," Harvin said. "As a team, we didn't execute on offense, defense and special teams."

Defensively, the Vikings have allowed quarterbacks to complete seven of eight passes for 114 yards on second-half third downs. Five of the completions went for a first down and one was intercepted. The defense also gave up running first downs on third-and-2 and third-and-1.

Unlike the offense, however, the defense got a boost from a new face on Monday. Five-time All-Pro defensive tackle Kevin Williams is back from his two-game StarCaps suspension. He spent the first two weeks like a lot of frustrated Vikings fans.

"I was here on the couch," he said. "Yelling at the screen."

Louder in the second half, of course.

"The first half, we look like world beaters," Williams said. "We've got to figure out how to keep that going over the second half."

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