The Germany international has missed almost all of pre-season due to a heel injury, but he returned for Tuesday's match at the Weser Stadion to help the northern Germany side recover from the shock of their DFB-Pokal elimination at the hands of third division side Heidenheim last weekend.
Mertesacker is therefore a candidate for the Bremen defence in their Bundesliga opener against Kaiserslautern this weekend.
"Even I was surprised at how dynamic I looked tonight [Tuesday]," said the 26-year-old on his club's website.
"To be totally honest, over the last year, I never felt as good physically as I did today [Tuesday].
"I never held back and that is an indication that I have got the injury totally out of my mind and this was a good warm-up for me."
Bremen coach Thomas Schaaf will be delighted to have the giant defender back after seeing his side crash out of the cup at the first hurdle last weekend.
Fellow central defender Sebastian Prodl was also back in action as he took Mertesacker's place for much of the second half after an even longer absence of almost four months due to a gluteal muscle injury.
"I am glad things worked out," said the Austrian. "It certainly gives me more mental strength after these very difficult last four months.
"I must admit that I was nervous, but it was great to be back on the pitch again."