There were three places up for grabs at The Renaissance Club and with third round joint leader Thomas Detry targeting his maiden European Tour title and Australian Min Woo Lee also in contention, it left Senior battling with Wade Ormsby for the remaining slot at the second Rolex Series event on the 2021 Race to Dubai.
Senior, who held the lead after the first two rounds before a 73 on Saturday, looked in pole position after a brilliant front nine yielded five birdies which propelled himself into co-leadership with Detry and fellow Englishman Matt Fitzpatrick.
Another birdie at the tenth saw the World Number 353 lead outright but bogeys at the 12th and 16th sandwiched a gain at the 14th to see him at the clubhouse at 14 under.
As the leading pack on the course improved to 17 under par, Senior had a nervous wait as Matthieu Pavon threatened to derail the Englishman's trip to Royal St George's Golf Club before Australian Ormsby became his next challenger.
Ormsby began the day on 11 under and traded a birdie at the second with a dropped shot at the sixth but the 41-year-old made his move with the 23rd ace of the 2021 Race to Dubai.
He hit the perfect tee shot at the 204 yard par three 12th and, after a couple of hops, found the bottle of the cup to propel him to 13 under.
As a result, Ormsby won a bottle of 10-year-old Glengoyne whisky for every yard of the the 12th hole, leaving him with plenty to celebrate.
A weather delay of 90 minutes also prolonged Senior's anxious wait, with Ormsby on the 16th fairway following his tee shot but he could not improve his score as Senior booked his fourth Open Championship appearance in five years.
"It would mean everything. 32,000 fans, playing in front of a home crowd. I've played three of the last four," he said.
"It would be great to play another Open, especially now I've found a little bit of form and my game feels like it's moving in the right direction. So there's no reason why I couldn't go there and have a good week as well.
"I'm going to head home now. Probably go around to my friends and watch the game and if England win, I mean, what a great weekend it's been for me and I'm really looking forward to watching the game."
Lee began the day three shots off the lead with compatriot Ormsby but he surged into tournament contention thanks to a birdie blitz from the the third.
He holed out six straight gains to climb to 17 under, where he held the outright lead for a short period, and added another birdie at the 16th to force a play-off with Detry and Fitzpatrick, where he prevailed to ensure he played in his maiden Major Championship.
"I didn't really think about The Open much, I just needed to do well here first and it is a bonus being there but it's awesome,"
"I've been playing really good in recent weeks and just trending, a few mistakes over the whole tournament and wasn't able to capitalise so it's awesome to do it here. I'll enjoy this one."
Detry will play in his fourth career Major tournament but it will be the first time at the Open Championship in Sandwich.