On the heels of a first Triple Crown winner in 37 years, there are now grand slams in play in both men’s golf and women’s tennis. Jordan Spieth heads into the British Open halfway to winning all four of golf’s majors in one calendar year, something that’s never been achieved before. Serena Williams, meanwhile, only needs to win the U.S. Open to become the first woman to capture a calendar year grand slam since Steffi Graf in 1988. One of these achievements would be monumental. But only one of these athletes is likely to pull it off.