Roger Federer knows he ain't gonna be the best forever ... and he's revealing to TMZ Sports his biggest threats to the throne -- and one of the guys just helped Fed break a huge record!!
We spoke with the 38-year-old superstar out in NYC and asked straight-up which young guns are giving him the biggest headaches ... and Federer name-drops 2 dudes he's got in the rearview -- 22-year-old German Alexander Zverev and 21-year-old Stefanos Tsitsipas from Greece!!
Of course, Federer and #7 Zverev just broke a major attendance record when more than 42,000 people showed up to watch their exhibition in Mexico City over the weekend.