The resurgence of Rafael Nadal and Roger Federer this season is all about turning a liability into an asset.

If tennis were investing, these two would be the master traders of 2017, the best on tour at taking a losing stock and turning it into a winning one.

The stock in focus is second serves.

Second serves are generally underrated, underperforming and under the radar when it comes to the discussion of tennis greats. Second serves for most of the tennis market are a speculative stock, but not for Nadal and Federer.

Nadal, who recently returned to the No. 1 ranking, and Federer, who is ranked third, have hoarded the Grand Slam singles titles this year and are among the favorites at the United States Open, which begins on Monday. They have never played each other in Flushing Meadows.