Miami, Mar 28 (IANS): Spain' Garbiñe Muguruza retired from her fourth-round Miami Open tennis match against Denmark's Caroline Wozniacki due to a heat-related illness on Monday.

The 23-year-old Muguruza called for the trainer following a gruelling first-set tie-breaker that she lost 6-7 (1-7) to the Dane at the Crandon Park Tennis Center in Key Biscayne, Florida. Tournament doctors checked Muguruza's pulse and temperature, reports Efe.

After Muguruza, the world No. 6, was treated for several minutes, the chair umpire announced that the 2016 French Open champion was retiring due to illness.

Muguruza did not show any visible signs of being sick during her hard-fought first set against the 26-year-old Wozniacki, who is ranked 14th in the world.

The Spanish No.1 once again came up short in Miami, where she also reached the fourth round in 2012, 2013 and 2016.

Muguruza was broken in her first two service games, blowing a 40-0 lead the second time she was on serve.

The Spanish star's struggles were highlighted during her second service game, when she missed an easy overhead smash at the net.

Wozniacki raced to a 3-1 lead and then held on serve before Muguruza rallied, taking the next four games and a 5-4 lead.

Muguruza failed to put away the Dane, who ran away with the tiebreaker.

The Spanish star retired a few minutes later.

Wozniacki, a former world No. 1 and winner of 25 WTA Tour titles, will face the Czech Republic's Lucie Safarova in the quarterfinals.

Safarova beat Slovakia's Dominika Cibulkova 7-6 (7-5), 6-1 in the round of 16 on Monday.