Elina Svitolina of Ukraine returns the ball to China's Wang Qiang (unseen) during their match on the second day of the WTA Dubai Duty Free Tennis Championship at the Dubai Tennis Stadium on February 21, 2018. Pic/AFP

Defending champion and top seed Elina Svitolina toiled for more than two hours to reach the Dubai quarter-finals while second seed Garbine Muguruza had to wait until 2 a.m. to complete her opening victory.