The phrase "fierce rivals" refers to two competitors who are extremely aggressive and determined in their efforts to outdo one another. They have a strong desire to succeed, often at each other's expense, and will stop at nothing to gain an advantage over the other. Their rivalry is intense, passionate, and sometimes even bitter, as they strive for superiority in whatever field or market they are competing in. The phrase implies that these two rivals have a deep-rooted competition with each other, where neither side will back down easily, making them formidable opponents to one another.