By an ace is a phrase that describes a narrow or lucky victory. Alternatively, you could use the synonyms "by a hair", "by a nose", or "by a whisker" to describe a close win. These phrases are often used in sports, such as horse racing, boxing, and tennis, to describe a victory that was won by a small margin. Another synonym for by an ace is "by a slim margin," which also implies a close or narrow win. Regardless of the synonym used, they all convey a sense of excitement and suspense, as the final outcome was determined by the smallest of margins.