The California tournament has reached the semi-final stage where Roger Federer faces Borna Coric before Juan Martin del Porto takes on Milos Raonic this evening.
Federer and Del Potro are expected to make it through to the final as they are the only top 10 players left in the competition.
If they do progress as expected then Shiras says he can only foresee one outcome.
And that is a victory for the world No 1 Federer as he is currently on a career-record unbeaten run of 16 games, which would stretch to 17 if he defeats Coric.
“Roger is the man to beat at the minute,” Shiras told Sky Sports.
“I know Del Potro is playing some really good tennis but I can’t look past Roger to win the competition.”
Federer is currently on an impressive winning run which has seen him record a 100 per cent start to 2018.
The 36-year-old lifted the Australian Open in January before overthrowing Rafael Nadal at the top of the ATP World Ranking by clinching the Rotterdam Open.
And ahead of his semi-final clash with Coric today (5pm), Federer says he is feeling confident.
“I think I have got to be playing very aggressive tennis as much as I can,” Federer told ATP World Tour.
“In windy and swirly conditions it is not always easy but I must focus on my own serve first.
“I have to try and get opportunities in the return and then mix it up and make them come up with passing shots.
“It is pretty simple, but not always easy to do.”