Birthday parties aren't pity parties, but Rafael Nadal isn't exactly celebrating aging.

The 31-year-old Spaniard scored his 50th career US Open victory today and will face 19-year-old Andrey Rublev for a spot in the US Open semifinal.

Watch: Nadal Rampages Into US Open Quarterfinals

Asked to identify the best thing about being 19, Nadal said he longs for his teenage days.

"Best part of it, of course if I can come back to 19, I take it," Nadal said with a smile.

The two-time US Open champion said ever since he was a child, he's wanted to stay forever young.

"Of course, better be 19 than 31. My opinion," Nadal said. "I always wanted to be young. Even when I was 8 years old, I was not very happy to when I was, my birthday, to be 9. Still the same. I am 31, and I am not happy when my birthday going to be 32. I am happy being young, no?"

While Nadal has turned back time regaining world No. 1 nine years after his initial rise to the top, he joked he hasn't yet figured out how to stop time.

"I don't want to get older," Nadal said. "For the moment, I didn't find the way to stop that watch." 

Photo credit: @USOpen