A cup tool includes a cup tool tube having a threaded upper end for connection to a high-pressure mandrel, an outer surface over which an elastomeric cup is slidably mounted for reciprocal movement from an unset position for entry into a wellbore to a set position in which an annular gap is obstructed to contain fluid pressure below the elastomeric cup. The outer surface of the cup tool tube has a lower region of a first diameter and an upper region with a second, larger diameter and a tapered region between the upper region and the lower region. The elastomeric cup includes a lip seal that rides against the outer surface of the cup tool tube, and seals against the tapered region of the cup tool tube to provide a high pressure seal between the cup tool tube and the elastomeric cup in the set position.