In Star Wars: Episode I - The Phantom Menace, Darth Maul is bested by the Padawan Obi-Wan Kenobi despite killing Jedi Master Qui-Gon Jinn only moments before. An instant fan favorite, Maul was a fearsome warrior with a deep connection to the dark side of the Force. Belonging originally to the Nightsister clan on Dathomir, Maul was taken as a child by Darth Sidious to be raised as his (first) Sith apprentice.
Best known for his appearance in The Phantom Menace, Maul's story has been continued in books and television shows wherein he creates the Crimson Dawn criminal empire after renouncing his last vestigial ties to the Sith. Maul encountered Obi-Wan Kenobi a number of pivotal times throughout his life, and was finally struck down by the Jedi on Tatooine as Galactic Civil War loomed large over the galaxy.
During the Invasion of Naboo at the climax of The Phantom Menace, Maul first kills Qui-Gon Jinn, then is defeated when Obi-Wan Kenobi bisects him with a shockingly non-fatal blow. Obi-Wan was by all s a gifted apprentice, but overcoming such an unprecedentedly deadly foe was a surprising success. A possible interpretation that has been suggested is that, in reaction to the death of Jinn, Obi-Wan briefly and unintentionally tapped into the dark side of the Force. The heightened aspects that his fighting takes on in that moment are similar to the power "focused rage," a recently-introduced dark side ability first ascribed to Kylo Ren, but which is similar to the former "Force rage" power present in Star Wars Legends. This interpretation could be ed by the fact that, years later when the two met on Raydonia, Obi-Wan chose to retreat rather than lean into Maul's goading, having matured in the intervening decade.
Although Obi-Wan was more orthodox than his master, he was still a Padawan during the Invasion of Naboo and remained susceptible to the brief pull of the dark side, especially in such a traumatic moment. Qui-Gon was also somewhat infamous among the Jedi Order for eschewing some of their tenets in favor of adhering to his own personal views, and while this has never been presented as sinister, a more lax view on the separation between the light and dark sides sides of the Force may have found its way into Obi-Wan's worldview. Qui-Gon was, after all, trained as a Padawan by Count Dooku, who would later commit fully to the dark side.
There are, of course, other possible explanations for the outcome of The Phantom Menace's final duel. In lightsaber combat, Obi-Wan was a skilled practitioner of Form III, meaning he fought more defensively than his master, which proved prudent. As for why Maul could defeat Qui-Gon in Star Wars, Maul had briefly been exposed to his fighting style during their introductory skirmish on Tatooine, so he may have been slightly more prepared for him than Obi-Wan, of whom he had no prior knowledge. One can only speculate about the full extent of Obi-Wan's interior experience in the Plasma Refinery Complex, but even if he did tap into the dark side of the Force, the moment that he takes to center himself while dangling above the reactor pit shows him firmly rooting himself in the discipline of the Jedi, and it is this which ultimately enables Obi-Wan to defeat Maul.
