The Williams Sisters’ rivalry is highly unusual and utterly remarkable. They are two of the greatest tennis players of all-time from the same family playing in the same era. And with all of their fiery competitiveness on the court, they’re still incredibly close off of it. They began playing against each other professionally in 1998 and have met 23 times on the court, with younger sister Serena besting Venus 13 of those. At the height of their dominance, the two played each other in four straight Grand Slam singles finals, with Serena winning all four (even though Venus still leads her sister in all-time career tennis titles with 43). But the most remarkable aspect of their rivalry may be that despite their competitive nature, they’re also the No. 1 ranked doubles pair. Read “The Power Game,” the TIME cover story on the Williams’ Sisters from the Sept. 3, 2001 edition.