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.
Federer beat Zverev in the exhibition, but Tsitsipas got the best of Fed in the ATP World Tour earlier this month ... before going on to win the whole thing.
42.517 Fans sahen in Mexiko-City, wie #Federer 3:6, 6:4, 6:2 gegen #Zverev gewann - neuer Rekord für die größte Zuschauermenge bei einem Tennismatch! Bisher lag dieser bei 35.681 Fans, die 2010 bei einer Exhibition in 🇧🇪 zwischen Kim Clijsters und Serena Williams dabei waren. pic.twitter.com/MmqRMeY04X