Canadian teenager Leylah Annie Fernandez caused a huge upset at the Monterrey Open by knocking out fifth seed Sloane Stephens to advance through to the quarter-finals.

The 17 year-old, who is coming off the back of a run to the Mexico Open final where she was beaten by Heather Watson, defeated her American opponent 6-7 (4) 6-3 6-3.

After two breaks in the opening two games, the pair held their serves for the remainder of the first set before Stephens edged Fernandez in the tie-break.

But that did not deter world No 126 Fernandez as she broke twice at the end of the second set to send the match to a decider.

And the dogged nature of Fernandez's play paid off as she raced into a 4-1 lead in the third set and served out the match to shock the 2017 US Open champion. 

"It was a great match. I was a little nervous in the beginning. She's a great player and she's won a Grand Slam. I knew it wasn't going to be easy.

"I tried to use everything that I've trained, as in forgetting about the past and focusing on the present, and that's helped me a lot. I've done a lot of mental training since I was very young and it's paying off now."

Next up for Fernandez is a match against either No 1 seed Elina Svitolina or Belarusian Olga Govortsova.

"It's going to be another tough match. Svitolina, she's a great player. She's won many tournaments and I've seen her play on TV," she added.

"I'll just go through my training tomorrow, maybe fix a few things, have a game plan and hope for the best, and prepare for the worst."