By Greg Stutchbury WELLINGTON, March 21 (Reuters) - Martin Guptill could not have found a better time to rediscover his form as he scored a World Cup best 237 not out on Saturday to lead New Zealand into the semi-finals against South Africa. Guptill's innings, which eclipsed Chris Gayle's 215 from earlier in the tournament, anchored New Zealand's 393 for six that gave the World Cup co-hosts a 143-run victory in their quarter-final against West Indies. The score was the first one-day international double century by a New Zealand batsman and followed the 105 he scored against Bangladesh in the final pool game on March 13. "I'm still not really sure what happened today," Guptill told reporters. "It hasn't sunk in yet. "I am incredibly proud of what happened today and hopefully moving forward we can win another game and then another one after that." A firing Guptill has shortened the odds of Brendon McCullum's side becoming the first from New Zealand to lift the World Cup, though until last week the 28-year-old seemed to be struggling to build an innings. Prior to the Bangladesh game he had been getting starts. He made 49 in the opening game against Sri Lanka and then was in double figures in the next three matches before he scored 57 against Afghanistan. That released the pressure valve and Guptill began to do what he does best against Bangladesh. Play straight, work the ball into gaps to turn the strike over and, when it was full, loft the ball in the arc between wide mid-off and deep mid-wicket. He repeated the recipe on Saturday against West Indies. The fact Guptill was the first New Zealander to pass 200 in a ODI, was not really a surprise. He scored 189 not out against England in 2013 and he said on Saturday that looking back at that innings, he felt he could have achieved the mark then. "I did muck around in the middle overs of that innings but I wouldn't take it back at all," Guptill said. "To set the record and then break it again today is pretty amazing and I just have to try and do it again I guess." Consistency has been Guptill's biggest problem in the past and he turned to New Zealand's best batsman in modern times, Martin Crowe, for help. Crowe stripped it all back to basics. Move your feet. Play each ball on its merits. Play straighter. Wait to hit the ball square until you were in. He said Crowe, who has terminal cancer, had sent him a message on Friday to just put the ball into gaps. Guptill, who lost three toes in an industrial accident as a teenager, was roared off field by the 30,268 crowd at the ground and he admitted that he had never felt he would experience that feeling. "It's pretty cool. I've never heard anything like that before." (Editing by Amlan Chakraborty)