Australian qualifier James Duckworth has been left hanging as John Isner, fresh off a seventh ATP title, continued on his winning ways at the Washington Open.

Duckworth was trailing South African seventh seed Kevin Anderson 3-6 6-3 4-5 when rain washed out their second-round match on Wednesday.

The pair will resume on Thursday, with Duckworth looking to join countrymen Bernard Tomic and Marinko Matosevic in the third round.

Isner, who edged Anderson in the Atlanta Open final on Sunday, fired 19 aces to beat qualifier Alex Kuznetsov 7-6 (7-2) 7-6 (7-4) at the joint ATP and WTA event.

Eighth-seeded Isner was lucky to progress before rain washed out most of the day’s matches.

That included tournament openers for Argentine top seed Juan Martin Del Potro, the 2009 US Open champion and a two-time Washington winner, and second seed Kei Nishikori, who received first-round byes.

As well as Duckworth’s match, German third seed Tommy Haas led American qualifier Tim Smyczek 3-6 7-5 1-0 when play was suspended.

The total washouts for Nishikori, ranked a career-best 11th this week, and Del Potro mean they will likely have to win five times in four days to take the title.

That bolsters Isner’s hopes for back-to-back trophies, despite a tough title run last week.

Big-serving Isner won six of seven tie-breakers he played in Atlanta, but dropped one to Anderson in the final and only won the last-set tiebreaker after saving two match points.

“The tank isn’t completely full, but I’ve got a lot of confidence,” Isner said. “I’ve been able to focus and play well in the tight moments. Using my weapon, my big serve, puts a lot of pressure on opponents.

“I feel like I can get sharper as this event goes along.”

Isner will play India’s 129th-ranked Somdev Devvarman on Thursday for a spot in the quarter-finals.

On the women’s side, Germany’s Andrea Petkovic upset sixth-seeded compatriot Mona Barthel 6-2 6-2 to reach the last eight in the only match completed.

“I was happy to get off to a great start and keep it up,” said Petkovic, who has been nagged by injuries so much this season that being near 100 per cent “is a nice feeling I almost forgot about.”

Petkovic said she was so happy to be healthy and playing again that “even the losses don’t hurt as much as they used to.”

Three other second-round matches were postponed to Thursday, including German top seed Angelique Kerber’s meeting with American Melanie Oudin.