This map was drawn by Japanese general staff in 1928.
Russian 6th corps is in Ortelsburg. But, shortly after German 17th corps nears it they retreat outside the border. On the other hand 13th corps and 15th are very active almost turning German left.
Germans call up 1st reserve corps (Below, 1st reserve division and 36th reserve division), Goltz territorial division, Ungel group, to side with 20th corps and 3rd reserve division. 1st corps waits for their timing. Next to 1st corps 5th mixed territorial brigade knocked out Russian cavalry two days before.
German encirclement was accomplished by 1st corps going straight east to take Willenberg and 17th corps going down to the south, Willenberg.
Ungel group consists of 20th mixed territorial brigade (garrisons at Thorn fortress) and Senmern territorial brigade(garrisons at Graudenz, Kuerm and Marienburg plus six reserve battalions accompanied by three training artillery squadrons). In reality these territorials were regarded before the war as not adequate for the battle. Golz division are territorials coming from Jutland peninsula where they were positioned to repulse British invasion.
However, German territorials and nation defence forces fought very well. 5th mixed territorial brigade, 70th and 6th famous for their heroic battle near Lake Great Bessau also participate in the battle of Tannenberg.
In total, German strength is around equal to 14 and half divisions. So, Germans outnumber Russians in the decisive battle field. After all military efficiency of Germany depends on territorials and nation defenders to some extent. On the other hand Russians left their reserve divisions behind.
Russian method in mobilisation was quite different from Germans or French. They have put "advanced disposition" into effect. It means that corps in Warsaw military district as well as in Vilna near Germany are fully equipped in war time level and mobilised in manpower even in peace time. So, it is easy for them to rush into East Prussia. On the other hand reservists come from Petersburg which is one of three major concentration cities including Moscow and Kiev. Russian rail capacity is not enough to make diagrams to follow military necessity. Reservists and other important equipments such as horses and guns have to be transported by fixed diagrams.
In Russia generals put ideas and railway decides all. So, are reservists. Russians make reserve divisions in order of arrival. As usual, Russian officers do not know where their soldier come from. And Siberian rifle brigade are often constituted by Volga comrades. It takes a long time to transport field guns and engineering equipment. It is easy to organise active corps but it is another to make reserve divisions.
That is why Russians can not dispose reserve divisions quickly. However, nearly the same thing happens to French but they are sensitive enough to get things flexible by lending equipments with each other. Russians also have some quantity of unmovable heavy and sometimes old guns at fortresses but they do not bring them out. Russians deplore to call these guns as artillery museum. But, Germans even use guns for training.