STANFORD, CA, USA - No.6 seed CoCo Vandeweghe booked her place in the second round of the Bank of the West Classic in Stanford on Tuesday after just 31 minutes as Ajla Tomljanovic retired due to injury.

The 2012 runner-up Vandeweghe had won the first set 6-2 when Tomljanovic retired.

"It was disappointing to have a retirement today, but I thought I was playing well to get to the point of winning the first set," Vandeweghe said. "It's just improving on what I want to do for the summer hard court series because this is the moment I love to play - the summer hard court series." 

The rest of the day's action saw Americans go 3-1 overall, as qualifier Caroline Dolehide stunned Naomi Osaka, 6-4, 6-2 to begin the day on Stadium court.

The 18-year-old American, ranked nearly spots behind the 19-year-old Osaka in the rankings, broke four times in the match to score her first WTA main draw win in one hour, 23 minutes.

Also advancing was 2016 Bank of the West Classic semifinalist Alison Riske, who rallied from a break down in the second set to see off Poland's Magda Linette, 6-2, 6-4.

The wins for Vandeweghe and Riske see them move one match away from a repeat of their Stanford quarterfinal from a season ago, where Riske led by a set before Vandeweghe suffered and ankle injury and was forced to retire.

Vandeweghe will face Stanford alumna Nicole Gibbs in the second round, another rematch from last year's event, while Riske faces No.4 seed Anastasia Pavlyuchenkova. Gibbs won the first set against Vandeweghe in the same round a season ago, before the 2012 finalist rallied to win in three sets. 

"I played her here last year, but other than that, it's more just focusing on myself and what I can do well," Vandeweghe said. "That's kind of how I'm going to win matches."