Montgomerie, five times the rankings winner, took advantage of good early playing conditions to fire a brilliant first-round 65, seven under par, at the Linde German Masters in Cologne.
That put the pressure firmly on Westwood, who is just pounds 22,000 ahead of the Scot in the race for the end-of-season title - there is pounds 166,660 on offer for the winner in Cologne.
The 25-year-old Worksop golfer responded superbly to record 66, just one behind Montgomerie, Vijay Singh and Maidenhead's Van Phillips.
Westwood's score was matched by Richard Boxall, Ian Garbutt, Jarmo Sandelin of Sweden and home favourite Sven Struver as the field made the most of a relatively short course. Darren Clarke, second in the current money table, managed only 69 as he complained of missing too many putts.
Montgomerie had no such problems - rating his putting nine out of 10. "It's not right by any means, but I started to putt a lot better and see the lines that I didn't at the start of the year," he said.
Westwood was equally happy with his round, which began with four straight birdies, all from within 20 feet, and contained just one dropped shot. "My back is not giving me any trouble and, although I'm tired, I had two day's rest and I think that's helped," said Westwood, who finished joint last in the Lancome Trophy in Paris.
Playing partner Singh had no problems adapting to the conditions. He had seven birdies in total - he was three under after three - and no bogeys and looked in ominous form ahead of the defence of his World Matchplay title at Wentworth.
Nick Faldo was not so happy with his opening round of 71 after another frustrating day with the putter. "I pushed a three iron into the water for a double bogey on the ninth and never felt comfortable on the front nine," said Faldo. "I hit it better on the back nine but only holed one long putt all day on the 14th."
Justin Rose fared even worse in his last chance to earn enough money to avoid going to the Tour qualifying school in November.
The 18-year-old needs at least a top-four finish but slumped to a four- over 76 and now looks certain to face the trials of the qualifying in Spain.Reuse content