Simona Halep's hopes of becoming world number one were dented as she was forced to withdraw from her quarter-final match at the Citi Open due to a heat-related illness.

Had Halep made the final in Washington DC, the top spot in the rankings - currently held by Karolina Pliskova - would have been up for grabs at next week's Rogers Cup.

However, the Romanian retired from her quarter-final with Ekaterina Makarova after one game of the third set, on a scorching hot day in Washington DC.

Halep won the first set 6-2 and looked well placed to move into the last four. Yet Makarova drew level by taking the second 6-3 and the Russian then claimed an early break at the start of the decider.

At the changeover that followed, Halep called for the tournament doctor before deciding she could not continue.

Makarova will face Sabine Lisicki or Oceane Dodin in the semi-finals.