After taking the first set in 39 minutes, Watson was broken twice in the second as the American levelled.

Seventh seed Watson broke twice in the decider before serving out the match and will play China's Xiyu Wang next.

In the men's event, fellow Briton Kyle Edmund was defeated 6-4 6-3 by American world number 35 Taylor Fritz in his quarter-final.

Fritz, who fired down 11 aces in the match, broke at 5-4 to take the first set and then broke the world number 44 twice more in the second.

Spain's Rafael Nadal eased into the last four with a 6-2 6-1 win over South Korea's Kwon Soon-woo. The 19-time Grand Slam singles winner will now play Bulgarian Grigor Dimitrov, who defeated third seed Stan Wawrinka 6-4 6-4.

If Nadal wins the Acapulco title and Novak Djokovic loses his semi-final against Gael Monfils at the Dubai Tennis Championships, the Spaniard will regain top spot in the world rankings.

Briton Watson, now ranked 69 in the world, said of her victory over McHale: "I knew I had to be aggressive because she wasn't going to give me any freebies."

When asked about their friendship, she added: "We've been hanging out here every day. We've got both of our mums here, we both went to the party together and have practised with each other.

"It's tough [playing your friend], but we're both used to it."