Mertesacker was criticised after apparently ducking beneath the ball to allow Martin Skrtel to net a 97th-minute equaliser for Liverpool in their 2-2 draw with Arsenal last Sunday.
But Wenger says the former German international -- he retired from international football after winning the World Cup in July -- was not to blame for his side's failure to hold on and claim three points.
Wenger concedes Mertesacker has struggled to find form during the opening months of the campaign following the World Cup in Brazil.
But the Frenchman is convinced that the 30-year-old defender is nearing a return to his best form and will only get better when Laurent Koscielny, his regular centre-back partner, returns from an Achilles problem next month.
"I believe he's on the way up
He had a difficult start to the season, but he's been much sharper lately," Wenger said.
"Many people had a go at him for the corner at Liverpool, but I looked at it and he had no chance to get that ball in the zone where he was.
"It's not easy for him
When Koscielny comes back and with Mathieu Debuchy being back, we will be more stable
Even in midfield we need to get our players back and to defend better.
"We had no stability at the back and when you look at the number of players who played centre-back this season, it's not easy."
The failure to beat Liverpool means Arsenal trail leaders Chelsea by 15 points going into the festive programme and now have their sights set on forcing their way up from sixth and into the Champions League places.
Striker Olivier Giroud will be expected to make a significant contribution if Wenger's side are to progress during the second half of the campaign.
The France international has scored twice in three games and is once again looking sharp after missing three months of the season with a broken leg, but he believes he must improve his strength in the air despite scoring an excellent headed goal against Newcastle United recently.
- Redknapp defends Wenger -
"It's surprising to hear, but headers aren't my best quality
I have the timing, but I still need to work on the technique," Giroud said.
"You always have a side you prefer more than the other one, so I try to work on it every week and it's something I can improve."
Arsenal will again be without midfielders Mesut Ozil, Aaron Ramsey and Mikel Arteta, but could welcome back Tomas Rosicky and Yaya Sanogo.
The north London club's difficulties this season have led to calls from a minority of supporters for Wenger to leave, but QPR manager Harry Redknapp has backed the Frenchman to turn things around.
"I can't believe why Arsene Wenger would get any stick
Arsenal are a good side
I still think they will finish in the top four," Redknapp said.
"They do it every year and I don't see why they won't do it again this year."
Redknapp's side are attempting to build on their excellent home form and continue their movement away from the bottom three.
QPR are keen to bring in more forward power during the January transfer window to support leading 11-goal scorer Charlie Austin, but Redknapp says he is only able to consider loan deals.
"We're not looking to spend money," he said.
"That's where we are at
We spent money to bring in players in the summer and now we have a good squad
But we can't be buying players or spending big money in the transfer window."
Yun Suk-young, the South Korea left-back, is missing with an ankle injury picked up during the 3-2 home win over West Bromwich Albion last weekend.
Source : AFP
Source: AFP