Coco Gauff said learning how to smile more often is behind her dazzling run at the US Open after she stormed into the semi-finals with a straight sets thrashing of Jelena Ostapenko on Tuesday.

The 19-year-old sixth seed became the first American teenager to reach the last four of the US Open since Serena Williams in 2001 after demolishing Latvia's Ostapenko 6-0, 6-2 in just 1hr 8min.

It was another polished performance that suggested Gauff is finally ready to land a breakthrough Grand Slam title, four years after she announced herself to the world with a magical run to the fourth round at Wimbledon in 2019.

Gauff's season has been transformed since she began working with veteran coach Brad Gilbert in July, weeks after she was left distraught by a first-round exit at Wimbledon.

Since Gilbert's arrival, Gauff has won titles in Washington and Cincinnati, where she also defeated outgoing world number one Iga Swiatek for the first time.

Gauff says one of the biggest changes Gilbert has made is to her mindset.

"I thought to play and win you have to be ultra serious and ultra focused, which is true, but also you still have to enjoy it," Gauff said after Tuesday's win.

"Honestly, the first meeting that I had with Brad before he started coaching with me, one of the first things he said is 'You need to smile more'. It wasn't really anything with my game or anything.

"I really took that, and when he said that I was a little bit surprised. I started to think and I was like, 'Yeah, I do.' That's something I'm trying to work on and continuing to do, and obviously I think it's helping my results."

Gauff meanwhile says she is flattered by comparisons to Serena Williams after matching the 23-time Grand Slam champion's feat in reaching the US Open last four as a teenager.

"I mean, being in any sentence with her is great. She's the greatest player of all time," she said. "I'm nothing close to that yet. I'm just really honored to be in the same sentence as her."

"She's my idol. I think if you told me when I was younger that I would be in these same stat lines as her, I would freak out."

Gauff will face either Czech 10th seed Karolina Muchova or Romania's Sorana Cirstea in the last four on Thursday.

- 'Crazy' scheduling -

On Tuesday's evidence, Gauff will head into the last four brimming with confidence after producing a superb display to overwhelm Ostapenko.

Ostapenko had produced the shock of the tournament in the last 16 on Sunday, ousting world number one and reigning women's champion Iga Swiatek in three sets.

But amid furnace-like heat on the Arthur Ashe Stadium, the inconsistent Latvian wilted in the face of a scorching performance from her American opponent.

Former French Open champion Ostapenko hit out at US Open scheduling after her defeat, stating she had not been given enough time to recover properly following her gruelling late-night victory over Swiatek on Sunday.

"I think it's really hard to recover from those night matches, because after beating world No.1, I went to sleep at 5 a.m.," Ostapenko said. 

"You sleep for maybe like seven, eight hours, but you don't completely recover. 

"I think it's a little bit crazy."

Gauff broke Ostapenko's serve three times in a lopsided first set which was over in 20 minutes, with the Latvian coughing 15 unforced errors to Gauff's two.

Ostapenko was soon in trouble in the second set, suffering a service break in the opening game when Gauff's sublime drop shot left the Latvian scrambling to the net.

However Ostapenko finally got on the board with a break of serve in the next game only to surrender that momentum in the next game when Gauff broke her for the fifth time for a 2-1 lead.

The next three games went with serve before Gauff turned the screw to break for a 5-2 lead that left the teenager serving for the match.

Although Ostapenko defiantly saved two match points, Gauff clinched victory with a forehand winner.

rcw/mw