WASHINGTON -  Top seeds Dominic Thiem and Simona Halep cruised into the round of 16 at the ATP and WTA Citi Open with straight-set triumphs Tuesday at the US Open tune-up event.

Austria's seventh-ranked Thiem, a French Open semi-finalist in June, waited out a rain delay then needed only 63 minutes to dispatch 96th-ranked Swiss Henri Laaksonen 6-3, 6-3. "It wasn't easy but I'm pretty happy with my performance," Thiem said. "It was my first match on hardcourt since Miami (in March) and that's always a little tricky."

World number two Halep, this year's French Open runner-up, took only 79 minutes to oust US wildcard Sloane Stephens 7-6 (7/3), 6-0. "It was a tough first set. I was missing a little bit too much and she made some shots," Halep said. "Didn't have the rhythm at the beginning, but I feel better now."

Thiem, who dropped only eight points on his serve and just one on 25 successful first serves, broke in the second game of the match and again in his rival's last two service games, taking the match when Laaksonen netted a backhand. "I don't have a lack of confidence but it's always nice to get that first win," Thiem said.

This marks the biggest event where Thiem has been top seeded, but his four nearest rankings rivals are also in the field -- Germany's Alexander Zverev, Japan's Kei Nishikori, Canada's Milos Raonic and Bulgarian Grigor Dimitrov. "I think there are five or six players to beat," Thiem said. "There should be four or five top seeds. I'm one of them."

But Thiem faces a possible third-round match Thursday against South Africa's Kevin Anderson, who has beaten him in all five of their career meetings, all on hardcourt and with Thiem taking only one set. "Of course I will think about it and I have to change something," Thiem said. "It's a very tough draw."

Second-seeded Nishikori turned back the determined challenge of American Donald Young 6-3, 4-6, 7-6 (7/5). Nishikori, seeking his first title of 2017, didn't face a break point in the third set, but he couldn't convert any of his eight break chances as Young forced the tiebreaker.

Nishikori finally sealed the win on his sixth match point after two hours and 25 minutes and with the clock ticking toward 2 am. Halep will next face Colombia's 115th-ranked Mariana Duque-Marino for a quarter-final berth as the 25-year-old Romanian keeps her focus on building her game for the US Open that starts on August 28 in New York.

"For first match on hardcourts, it wasn't that bad," Halep said. "My serve wasn't the best but I was OK. Everything went well. I can't say I have to adjust something. It was important I won today. It was important the second set was much better. The next round I have to be a little more aggressive. I have to come to the baseline and finish the points sooner."

Stephens has been back for only a month after missing nearly a year with a left foot injury, falling to 957th in the rankings. She remains frustrated as she tries to regain the form that saw her claim her first WTA title in Washington in 2015.

"It was a good first set," Stephens said. "I did well to do the best I could. I got a little frustrated in the second set. It's going to take a while to get that consistency back and get more matches under my belt."