The first of three all-American matches slated for the opening round at the Mubadala Silicon Valley Classic went the way of No.5 seed Alison Riske on Monday, after the freshly-minted Olympian overcame CoCo Vandeweghe in straight sets, 6-3, 6-4. 

From 3-0 down in the opening set, Riske won nine straight games en route to beating Vandeweghe for the fifth time in 12 career meetings and first time in over four years.

"I'm so thrilled. This year has been really challenging for me. Any time I start off a match [facing a deficit] like that, I'm thinking, 'Oh no, not this again.' I'm really thrilled that I was able to push through."

- Alison Riske

Efficiency on break points was in part the difference-maker in the 88-minute affair: Vandeweghe went just 3-for-12 on chances on Riske's serve, which included a pair of unconverted 0-40 leads. She was unable to stretch her first-set lead to 4-1 on the first trio, and Riske sealed victory by winning five straight points in the 10th game of the second set.

Former World No.9 Vandeweghe, back on tour after foot and hand injuries, also racked up 28 unforced errors to 17 winners, while Riske posted a tidy 11 winners to 12 unforced.

The all-time history between the two compatriots dates back to 2008, and the victory is just Riske's third overall match win this season after stop-and-start struggles with her own foot injury.

"Playing someone like CoCo who can take the racquet out of your hand, especially on the first couple of ball off the serve, off the return, you have to be ready to go from the get-go," Riske added.

"It took me a second to get used to it, but mentally, I hung in there and I stuck with it, and at the end of the day, that's the most important thing for me."

The World No.37 had been an abridged 0-2 on hard courts this year, having been beaten in the first round of both the Australian Open and last week's Tokyo Olympics. For a spot in her first quarterfinal since last January's Brisbane International, Riske will face Croatian qualifier Ana Konjuh, who rallied from 5-3 down in the final set to beat Czech Marie Bouzkova, 7-5, 3-6, 7-5.

The match saw 15 breaks of serve in all between the two players, who combined for 30 break point opportunities in a grueling 2 hours and 41 minutes.

"This is the first time I played her, so I didn't really know what to expect," Konjuh said after the match. "I don't think it was the best tennis from either one of us today, but on those days, it's really important to pull through and find a way to win. I think I managed that today."

Other first-round winners in Monday's day session were No.6 seed Petra Maric and Magda Linette, each of whom advanced by 6-3, 6-3 scorelines over Anastasija Sevastova and Lesley Pattinama Kerkhove, respectively.

No.6 seed Martic brushes past Sevastova in San Jose opener: Highlights

In the night session, China's Zhang Shuai eased to a 6-3, 6-2 win over British wildcard Emma Raducanu, the latter playing her first match since a breakthrough run to the fourth round of Wimbledon last month.

The 32-year-old veteran broke the 18-year-old's serve four times in all and sprinted out to leads of 3-0 and 4-0 in each set. Though Raducanu got the opener back on serve, she was unable to convert a game point for 3-3, and never got closer in the match despite battling throughout for 82 minutes.

The teenager couldn't convert a pair of break point chances early in the second - opportunities which would've seen her level at 1-1 - but later saved a hold out of a 0-40 deficit to get on the board in the fifth game.

Thanks to 29 winners - nearly twice Raducanu's 15 - Zhang snapped a six-match skid on hardcourts dating back more than a year.

"She's a really nice girl and plays really, really great tennis. I played really well tonight," Zhang said on-court after the match.

"I missed hardcourts so much because in the last three-and-a-half months in Europe, we were on clay and grass. I was so excited today."

Having lost in the first round in her three previous trips to the tournament in San Jose and at its previous location in Stanford, Zhang advances to a second-round meeting with No.3 seed Madison Keys. The pair have played six times with Zhang holding a 4-2 head-to-head lead, including the last three wins. 

Rounding out the night session with a victory was Caroline Garcia, who beat American qualifier Emina Bektas in the latter's WTA main draw debut, 6-4, 6-4.