On the southern end of the German line, Otto von Below's I Reserve Corps had flung out its cavalry to conduct the reconnaissance that Mackensen neglected. Finding no Russians, the corps turned to the northeast to strike what they assumed to be the enemy left flank. German pilots spotted the Russian IV Corps, but too late to warn Below that his reservist cavalry had completely missed 30,000 Russians moving quickly on a flank attack of their own.

The reservists recoiled at the Russian attack, and their cavalry belatedly tried to attack the oncoming enemy with little effect. The artillery didn't offer much help either, but surprisingly the Germans rallied from their initial shock and began to push forward. By mid-afternoon they had recovered all the lost ground and stopped the Russians from breaking through on their mission to turn the flank of the German XVII Corps.