Nah, it can be asymptomatic if it's not severe enough. So one possibility to explain what happened: five hours of tennis created enough water-loss that his body had a temporary shortage of bloodflow in the head region. Maybe the ophthalmic artery, who knows. Kinda like standing up too fast and getting lightheaded. Five hours is a very, very long time for tennis, and you have to make sure you eat/drink adequately during that time.
Once I got up too fast, and my vision was blurry for 15 seconds. Thank God I didn't faint. Remember to eat!