— Novak Djokovic, Roger Federer or Rafael Nadal has been US Open No. 1 seed in 16 straight years.
— From 2004 through 2019, 479 different players have competed in US Open men’s singles main draw.
— Djokovic, the World No. 1 and defending champion, is 33-1 in his last 34 matches at Grand Slam tournaments.
— Djokovic is 13-0 in US Open 1R. He is 53-0 in Grand Slam 1R since 2006 Australian Open (l. to Paul Goldstein).
— Djokovic is 17-0 at US Open and 75-1 at Grand Slams against opponents outside Top 75 (l. to No. 117 Denis Istomin in 2017 Australian Open 2R). The 3-time champion meets No. 76 Roberto Carballes Baena on Monday.
— Last time No. 1 seed lost in US Open 1R was 1990, when Alexander Volkov upset Stefan Edberg. No. 1 seed has reached US Open QFs or better every year after 2000, when Arnaud Clement beat Andre Agassi in 2R.
— Federer and Nadal have never met at US Open. But Federer and Nagal will meet on Monday… Sumit Nagal.
— Nagal, a qualifier, has never won a tour-level match (0-4). Federer has won 1,223 tour-level matches (1,223-266).
— If Federer beats Nagal, he will extend his record and qualify for 17th Nitto ATP Finals (2002-15, 2017-19).
— Federer is 18-0 in US Open 1R. He is 61-0 in Grand Slam 1R since 2003 Roland Garros (l. to Luis Horna).
— Federer is 18-0 at US Open and 60-2 at Grand Slams against opponents outside Top 100 (l. to No. 154 Mario Ancic in 2002 Wimbledon 1R and No. 116 Sergiy Stakhovsky in 2013 Wimbledon 2R). Nagal is ranked 190th.
— Ten American men play Monday, including Zachary Svajda, 16, who meets Paolo Lorenzi, 37, on Court 5. The 21-year age gap is biggest at a Slam since Ken Rosewall, 44, beat Cary Stansbury, 20, in 1978 Aus. Open 1R.
— Svajda is youngest US Open men’s singles player since Donald Young, 16, in 2005. If he beats Lorenzi, Svajda will be youngest to win a US Open men’s singles match since Michael Chang, 16, reached 1988 US Open 4R.
— Jack Sock has won US Open titles in boys’ singles (2010), mixed doubles (2011) and men’s doubles (2018). Sock seeks his first men’s singles win of 2019 season (0-3) when he meets Pablo Cuevas of Uruguay on Court 12.
— Reilly Opelka is making his fifth straight US Open appearance and his first in main draw (2015-18 qualifying). Following Opelka on Court 17 is fellow American Taylor Fritz. Opelka was best man at Fritz’s wedding in 2016.
FEATURED MEN’S SINGLES 1R MATCHES
ARTHUR ASHE STADIUM
 Novak Djokovic (SRB) vs Roberto Carballes Baena (ESP) — First Meeting
 Roger Federer (SUI) vs [Q] Sumit Nagal (IND) — First Meeting
LOUIS ARMSTRONG STADIUM
Prajnesh Gunneswaran (IND) vs  Daniil Medvedev (RUS) — First Meeting
 Stan Wawrinka (SUI) vs [Q] Jannik Sinner (ITA) — First Meeting
[Q] Marco Trungelliti (ARG) vs  Kei Nishikori (JPN) — First Meeting
Sam Querrey (USA) vs Juan Ignacio Londero (ARG) — First Meeting
 Fabio Fognini (ITA) vs Reilly Opelka (USA) — First Meeting
 Taylor Fritz (USA) vs Feliciano Lopez (ESP) — Lopez Leads 1-0
— ATP Media Info