Could Obi Wan Have DEFEATED Darth Vader?

In Star Wars a New Hope Obi Wan Kenobi Dies in a duel against Darth Vader on the Death Star. At the time Alec Guinness portrayed Obi Wan who seems weak and frail compared to Anakin Skywalker. However just two years earlier Obi Wan Kenobi managed to easily handle Darth Maul. So why did he seem so weak in this duel. Was he holding back? Could he have defeated Darth Vader?