Watch the WTA Acapulco LIVE on beIN SPORTS  

Kyrgios looked poised for a swift exit after losing the first set and calling for the trainer, complaining of illness against the 17-time grand slam champion in Acapulco on Thursday (AEDT).

But the maligned Australian produced an astonishing display, fending off nine break points in the marathon encounter – including five in the third set – before saving three match points.

Nadal then sent a forehand long as Kyrgios – who hit 58 winners and 49 unforced errors – was booed by the crowd following more than three hours of scintillating tennis.

Kyrgios handed two-time champion Nadal just his second defeat at the ATP 500 tournament as he earned a last-eight showdown against Stan Wawrinka.

There were five comfortable holds of serve in the opening set until Kyrgios – often nonchalant and erratic in his approach with flashes of brilliance – was too aggressive on serve.

A double-fault gave Nadal – who finished with 39 winners and 17 unforced errors – a break point and he did not need a second invitation as he broke for a 4-2 lead and the former world number one never looked back, racing away with the set.

Kyrgios sought the trainer at the end of the set and said he was feeling sick. As the trainer alluded to retirement, the 23-year-old replied: "If I forfeit, the media will blow it up... I'll be booed off the court."

He continued and emerged with a new lease on life, producing some show-stopping moments as Kyrgios dug himself out of a 0-40 hole to go 5-4 ahead in the second set, which he went on to win after blowing Nadal away in the tie-break.

The momentum was well and truly with Kyrgios, much to the annoyance of Nadal, who squandered another five break points in the sixth game of the deciding set.

Kyrgios looked out on his feet but refused to go away, as he somehow emerged from the jaws of defeat to claim his third win in six meetings with Nadal.