If Jelena Ostapenko can dominate her tennis opponents when she is unwell, then just imagine what she’ll do at full health.

The 19-year-old Latvian has progressed to the Auckland Classic semi-finals after overcoming flu to defeat American giantkiller Madison Brengle 7-5 6-3.

Waking up on Thursday morning with a sore throat and lost voice, the world No.44 nevertheless managed to roll over her opponent with pace and a thumping forehand.

In an even greater feat, she strolled out onto centre court an hour later for her doubles quarter-final, recording a win alongside Belgian partner Kirsten Flipkens.

The 19-year-old world No.44 will go on to the singles semi-finals, where she’ll face American 2015 semi-finalist Lauren Davis, as well as the doubles semis.

But it almost didn’t happen for the seventh seed.

“In the morning, I woke up and had a cold, so it was a bit difficult to play for me,” Ostapenko said with a hoarse voice.

“I don’t feel 100 per cent but I hope it’s going to be better tomorrow.”

Despite the victory, the Riga-based young gun didn’t have it all her own way in Auckland, having her first two error-strewn service games broken.

Down 3-0 early, Ostapenko returned fire to win five consecutive games and ultimately the set, before again storming home in the second set.

The triumph follows a romping first-round win over Kiwi local Marina Erakovic, as well as a clinical straight-sets win over Mirjana Lucic-Baroni.

Ostapenko said she was happy with her displays so far and would need to give even more to defeat Davis, who defeated Czech fourth seed Barbora Strycova.

But first, she’d need to win the battle against her own body.

“I won the match, so I’m glad,” Ostapenko said.

“I’ll try to do everything to feel better.”