Young American Jenson Brooksby claimed some revenge for last month's Hall of Fame Open final defeat to Kevin Anderson by beating the South African 7-6 (7-4) 6-3 in the Citi Open first round on Monday.
Brooksby, who received a wildcard for the draw, triumphed in one hour and 49 mnutes against the former world number five.
The 20-year-old American saved a set point at 5-6 in the first set, before winning in a tiebreaker off the back of winning six consecutive points having trailed 1-4.
Former City Open finalist Anderson, who sent down 10-1 aces, had defeated Brooksby to win the Hall of Fame Open last month.
World number 130 Brooksby, who had started the season outside the top 300, claimed the crucial break in the eighth game of the second set.
Brooksby advances to take on American Francis Tiafoe, fresh from competing at Tokyo 2020, as numerous big names enter the draw for the second round.
Kei Nishikori, who won in Washington in 2015, got past American Sam Querrey 6-4 6-3 to set up a second round meeting with Kazakhstani ninth seed Alexander Bublik.
American Jack Sock will take on top seed Rafael Nadal in the second round after he got past Yoshihito Nishioka in a walkover 6-7 (3-7) 4-0.
Former top 20 player Andreas Seppi triumphed in three sets against Yasutaka Uchiyama while Canadian tour veteran Vasek Pospisil got past Emilio Gomez in three.
Ilya Ivashka, Marcos Giron, Ricardas Berankas and Daniel Galan were the other first round winners on Monday.