Farfan latched on to a neat pass from Lewis Holtby to race free in the area and poke the ball beyond the advancing Heinz Muller in the 12th minute.

Schalke continued to press but could not find a second goal.

However, that will not worry them too much as they go top by two points ahead of second-placed Bayer Leverkusen's meeting with Borussia Monchengladbach tomorrow.

Schalke were on the attack quickly, with Marcelo Jose Bordon sending an early free-kick wide before Farfan's strike.

The hosts ramped up the pressure with three corners in as many minutes but wasteful finishing kept the lead at one.

Mainz did not get a sight of goal until the 28th minute, but Milorad Pekovic's free-kick was punched clear by Manuel Neuer.

Schalke had a slight wobble at the start of the second half, when Neuer was lucky to get away with a moment of uncertainty outside the area, but he managed to race back and get his hands on the ball before Mainz could capitalise.

The hosts were soon on the attack again, as Ivan Rakitic saw his shot deflected for a corner after a good run.

Kevin Kuranyi then hit a shot into the side netting as Mainz continued to hang on and stay in the game.

However, they could not take advantage of Schalke's profligacy as Srdjan Baljak fired over after a good run into the area in the 85th minute.

Kuranyi could have wrapped it up in injury-time, but he saw his shot deflected wide.