WASHINGTON - US top seed John Isner and second-seeded defending champion Ivo Karlovic advanced to the quarter-finals of the ATP Hall of Fame Championships with straight-set triumphs Wednesday at Newport, Rhode Island.

Isner, the 2011 and 2012 winner at the grass-court event, fired 13 aces in defeating Australian qualifier Sam Groth 6-2, 6-4. The hard-serving American improved to 3-0 all-time against Groth, who dropped prior meetings in Davis Cup ties this year and last year, with the 61-minute triumph. Croatia's Karlovic, 38, defeated American Denis Kudla 6-4, 6-4 to reach the final eight. Karlovic, seeking his ninth career ATP crown, lost the 2014 and 2015 Newport finals before downing Gilles Muller 14-12 in a third-set tie-breaker to win last year's championship match.

Isner advanced to a quarter-final matchup against fellow American Dennis Novikov, a 6-3, 6-2 winner over Canadian Frank Dancevic. Karlovic booked a quarter-final meeting with Germany's Peter Gojowczyk, who eliminated Russia's Konstantin Kravchuk 2-6, 6-4, 6-4. Germany's Tobias Kamke pulled the biggest upset so far, ousting French third seed Adrian Mannarino 7-6 (7/5), 7-5. He will next face Australian qualifier Matthew Ebden, who defeated sixth-seeded Slovakian Lukas Lacko 6-1, 7-6 (7/2). Also advancing was fourth-seeded Frenchman Pierre-Hugues Herbert, who outlasted Spain's Adrian Menendez-Maceiras 7-5, 2-6, 6-3. His next opponent American Bjorn Fratangelo, who ousted Australian Akira Santillan 7-6 (7/3), 3-6, 6-1.

HALEP, RAONIC TAKE WASHINGTON WILDCARDS: World number two Simona Halep and men's top-10 players Milos Ranoic and Grigor Dimitrov have accepted wildcard entries for the ATP and WTA Washington Open, organizers announced Wednesday.

The US capital's hardcourt event, which begins July 31, comes just ahead of the top US Open tune-up tournaments in Canada and Cincinnati for the US Open, which begins August 28 in New York. French Open runner-up Halep, a 25-year-old Romanian, stayed a career-best second in the rankings after Czech Karolina Plskova jumped to number one after Wimbledon and Germany's Angelique Kerber tumbled to third.

The return of 2014 Washington winner Raonic, who lost a Wimbledon quarter-final to eventual winner Roger Federer this month, gives the event its past four men's champions in the field. Three-time winner Juan Martin del Potro, whose most recent crown came in 2013, was already set, as were eighth-ranked Kei Nishikori of Japan, the 2015 Washington champion, and 13th-ranked defending champion Gael Monfils of France.

Bulgaria's 10th-ranked Dimitrov joins Nishikori, Raonic and Germany's 11th-ranked Alexander Zverev to give the event four of the world's top 11. Also accepting a wildcard was Sloane Stephens, who won her first WTA title at Washington in 2015.

Stephens, who has fallen to 901st in the rankings, missed 11 months after a left foot injury that required surgery in January. She made her comeback in a first-round loss at Wimbledon to fellow American Alison Riske. Stephens, who had not competed since a first-round Rio Olympics loss to Canada's Eugenie Bouchard last August, won three times last year before the injury. Bouchard, a 2014 Wimbledon runner-up, and Rio Olympic champion Monica Puig of Puerto Rico are also in the women's field.