Jack Sock was asked about his upcoming match with No. 11-ranked Kei Nishikori, a query that ended with high praise for Nishikori's level of play."Yeah, thanks for that," Sock shot back in a deadpan tone.No. 94-ranked Sock played pretty well himself Tuesday, beating No. 59-ranked Igor Sijsling 6-4, 6-2 in the first round of the Citi Open. That victory over Sijsling - the second-highest-ranked opponent Sock has ever beaten - sets up a second-round matchup with No. 2-seeded Nishikori."If I'm playing well, I think I have a shot at a lot of guys," Sock said.The 20-year-old American said he's still adjusting to life on the pro tour, but he was in good spirits after Thursday's match, signing autographs and posing for pictures with young fans."If you're winning, there's nothing better," Sock said.Sock added that he will play mixed doubles with Sloane Stephens at the U.S. Open in August.Another American, No. 107-ranked Ryan Harrison, upset No. 57-ranked Lleyton Hewitt 6-3, 7-5. No. 4-seeded Milos Raonic beat Samuel Groth 7-5, 6-4. No. 22-ranked Sam Querrey, the tournament's sixth seed, beat Denis Istomin 4-6, 6-3, 7-6 (9)."It was stressful out there," said Querrey, who began his postmatch news conference with a hearty yawn. "Just happy to get through it."On the women's side of the Citi Open, top-seeded Angelique Kerber cruised past Irina Falconi 6-2, 6-3 in the first round.On the advice of her coach, Kerber headed straight to the practice court for a 20-minute session."I think it was a good idea," said Kerber, who hasn't played a tournament since Wimbledon last month.With No. 15-ranked Stephens' loss Monday, No. 9-ranked Kerber is 17 spots higher than any player remaining in the tournament. The German will face the winner of the match between Melanie Oudin and Anastasia Rodionova.Taylor Townsend lost to Monica Niculescu 6-3, 6-0. Last year, Townsend became the first American girl to finish No. 1 in the year-end world junior rankings."I'm the youngest one here in the tournament playing, so I don't have any pressure really," said Townsend, who got a wild-card entry. "No one is expecting me to do anything. So, every win is a shock, basically."I have high expectations for myself and a lot of confidence and belief in myself. But basically, there's no pressure on me."Also, No. 40-ranked Madison Keys beat Michelle Larcher de Brito 7-5, 6-3.