The Ranktracker REST API (v1) is now publicly documented. It gives you API-key–authenticated access to your rank-tracking data:

• Domains — full CRUD, plus ranking history and share of voice
• Competitors — full CRUD per domain
• Keywords — track, read rankings, pause/resume, and stop tracking
• SERP — latest results and cached SERP HTML for a keyword
• Tags — organise keywords individually or in bulk
• Account — plan usage and quota

Head to the Quickstart to make your first request.