Munk Mortensen (age 19) just beat Renata Jamrichova (age 17) who won two Junior Grand Slams last year - Wimbledon and Australian Open
Renata has been doing well, but not as well as many expected - it shows how tricky it is even for the very, very best juniors to move up (whereas others who didn't lift the trophies in juniors find it a lot easier)
Its more common that you'd think in tennis and maybe more in Ladies tennis, where players relax a bit having got the first set and lose their serve and the set then re-focus for the final set.