There is a slim chance, yes.
In 2014, Erdoğan scraped together his first-round victory over what was a weak cross-party candidate for both the MHP and CHP, and the HDP's Selahattin Demirtaş. In 2018, he's facing at least four other candidates that will split his vote and at least take it to the second round. They're also strong candidates that will rally the bases of each party like never before.
When it comes to the second round - and it will - that's where millions of Turks will be hyperventilating in anticipation. I believe they can beat him with their strategy, and I simply refuse to believe he has the necessary votes from the AKP supporters alone, so then we get to see him extra cards out of his sleeve. Whether or not the opposition accept the inevitable corruption in his desperate bid to consolidate his position is the important factor, and I don't believe we've seen anything like this before. As I said above, they're strong candidates, and they can not only rally their voters to turn out, they can also move them to refuse his blatant corruption. That's when it's time for Turkey to take a stand against this shit once and for all.
It's also worth remembering that it's not just about who opposes him. The economy is an absolute mess right now, and if they sense he's weak (and this is a guy who can't stand to look even slightly weak), enough of his people might even jump ship and assist in ousting him.