In a match between 18-year-old Americans, No.6 seed Coco Gauff defeated qualifier Robin Montgomery 6-3, 6-3 in their first-round match at the San Diego Open.

"It was the first time playing someone younger than me in probably at least seven years," Gauff said. "It was definitely a different feeling for me."

Here are some takeaways from a match that spanned two days:

Successful return to tour for Gauff: World No.8 Gauff was playing her first match since she was stopped in the US Open quarterfinals by Caroline Garcia more than a month ago. But Gauff was not rusty playing her first match as a Top 10 player.

Gauff's serve was on point, with three aces and additional unreturnable serves helping her win 76 percent of her first-serve points. Gauff's superb defense helped extended rallies and forced Montgomery into errors.

Gauff came back from an early break down in the first set to win four straight and capture the opener.

Gauff also had to survive a 50-minute rain delay in the second set, followed by an overnight rain delay while up a break. But she remained unbothered and needed less than 20 minutes Wednesday to close out the victory.

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"I know Robin very well," Gauff said. "We went on a couple USTA trips back when we were like 8 years old. Pretty much friends ever since. She played really well. I expected a good level from her. ... I was pretty impressed with how well she was striking the ball. It was difficult for me.

"I told her at the end of the match, maybe next time we play, it's not in a first round match, maybe we'll play in a final."

Keep an eye on Montgomery: In her first match against a Top 10 player and just her fifth main-draw match at tour level, the left-handed Montgomery found a bevy of winners in the first five games, from deft drop shots to powerful backhands down the line as she broke for 3-2 to take the early lead.

The advantage did not hold as Gauff gritted her way into a dominant position, but Montgomery still looked strong in defeat, with only three fewer total winners than this year's Roland Garros finalist. Four double faults hindered Montgomery's cause down the stretch.

The match followed a breakthrough weekend for 2021 US Open Junior Singles champion Montgomery. She received a wild card into the qualifying, where she beat two Top 50 players, Zhang Shuai and Bernarda Pera, for the first Top 100 wins of her career.

"It was an amazing experience," Montgomery said after the match. "I'm really happy they gave me the opportunity to come through qualifying, and it was really great playing at this level."

Eyes on the prize: Gauff will continue to forge ahead for the rest of the season, where she is close to qualifying for the WTA Finals in singles (she is currently No.4 in the Race) and doubles (she and Jessica Pegula are No.5 in the doubles Race). One or both of those qualifications could happen this week if Gauff progresses far enough.

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First things first: Gauff will stare down a Grand Slam champion in the Round of 16 when she takes on Canada's Bianca Andreescu, the 2019 US Open titlist. It will be the first meeting between two players who came on to the scene with great acclaim as teenagers.

"I think [Andreescu] is a very aggressive player," Gauff said. "We all know she likes to step into the court and take balls early. I think it's going to be an interesting matchup."