John Isner, Gael Monfils and Bernard Tomic enjoyed straight-sets wins in the Citi Open second round.

A three-time runner-up, including last year in Washington, Isner sent down 14 aces in a 6-3 6-4 victory over James Duckworth.

It was a good day for the top seeds at the ATP 500 event, with Monfils and Tomic also among those advancing.

Monfils needed just over an hour to crush Lu Yen-hsun 6-3 6-2, while Tomic was tested early before beating Donald Young 7-6 (7-4) 6-3.

Of the 12 seeds in action, only Kevin Anderson and Viktor Troicki were beaten.

Anderson was edged out by Malek Jaziri 5-7 6-4 7-6 (7-3), while Troicki lost to qualifier Ryan Harrison 7-6 (7-4) 6-4.

It was relatively straightforward going for the rest of the seeds.

Jack Sock claimed a 7-6 (7-1) 6-4 win over Lukas Lacko and Alexander Zverev eased past American teenager Taylor Fritz 6-4 6-2.

Borna Coric was too good for Yuichi Sugita 6-4 6-4, Gilles Muller crushed Yoshihito Nishioka 6-4 6-1 and Ivo Karlovic served 19 aces in a 6-3 7-6 (7-4) win over Brian Baker.

Steve Johnson completed his 6-3 6-4 win over Adrian Mannarino and Marcos Baghdatis cruised past John Millman 6-2 6-4.