San Francisco Frontiers - Gauff digs deep to keep China Open title defence alive

"I did what I needed to do to win today," the 21-year-old American said.

"It was very physical," Gauff said. "We were going back and forth."

Gauff looked in control in the first set but the gritty Fernandez hit back in the second to force a decider.

The 25th-ranked Canadian survived a match point to level at 5-5 but Gauff finally clinched the 2hr 45min marathon when Fernandez netted.

The world number three next plays either Switzerland's Belinda Bencic or qualifier Priscilla Hon of Australia for a place in the quarter-finals.

In the men's draw, Italy's Lorenzo Musetti apologised on court after his match Sunday for an outburst at coughing fans earlier in the tournament.

"Well first of all, I want to say... a public apology," the 23-year-old Italian said in English after beating France's Adrian Mannarino 6-3, 6-3 to move into the quarter-finals.

Fans booed when he entered the court for the match and some were making loud guttural coughing noises.

"I totally understand your point and I... regret what I've said and what I've done, so sorry, thanks for understanding," he said.

The on-court interviewer, however, did not translate his apology into Chinese for the fans, only relaying Musetti's brief comments about the match.

In the women's draw, Germany's Eva Lys stunned world number 10 and former Wimbledon champion Elena Rybakina 6-3, 1-6, 6-4.

"I was trying to get a top 10 win the whole year," said the 23-year-old Lys, ranked 66.

"This is my first one. I'm very happy I finally got it done."

Poland's Iga Swiatek is the top seed at the WTA 1000 event after world number one Aryna Sabalenka withdrew.

Italy's Jannik Sinner is the top seed in the men's ATP 500 event being played alongside the women's tournament.

T.Martin--SFF