Hi Zohaib,
TestComplete does not support load testing functionality for more than a year (
http://smartbear.com/forums/f77/t75218/testcomplete-load-testing-end-of-life-effectiv). The previous version of the tool that supported load testing functionality supported traffic processing and modification.
Note that only http(s) traffic was supported.
Now all load testing functionality is moved into another tool called LoadUIWeb (
http://smartbear.com/products/qa-tools/load-testing). The tool works with http(s) and allows traffic modification.
There is also LoadUI tool (
http://smartbear.com/products/qa-tools/web-service-load-testing) that is intended for gui-less api testing. The difference between LoadUIWeb and LoadUI is described here:
http://loaduiweb.org/faqs/loaduiweb-general.html ("What is the difference between LoadUIWeb and LoadUI?" topic).
If LoadUIWeb does not suit your needs for some reason, you may consider SoapUI (
http://smartbear.com/products/qa-tools/web-service-testing-tool).
