WIMBLEDON, England--Left-hander Tom Gullikson of the U.S. upset 12th-seeded Miloslav Mecir of Czechoslavakia in five sets yesterday in the Wimbledon tennis championships.

Gullikson, from Palm Coast, Fla., beat Mecir 4-6, 6-3, 6-4, 6-7 (5-7), 6-3 in just under three hours on Court No. 1 at the All England Club.

Earlier, after a one-hour rain delay, Hana Mandlikova, the No. 3 women's seed, swept impressively into the second round with a 6-0, 6-1 victory over fellow Czechoslovakian, Iva Budarova.

Boris Becker, the 17-year-old West German who is considered the most dangerous nonsked in the draw returned to center stage and completed his unfinished mater against American Hark Plaster.

The big-serving Becker was leading two sets to one when darkness halted the match Wednesday at 2-2 in the fourth set.

It took the West German only 25 minutes to finish the job. He broke Pfister's serve for a 4-3 lead and held on for a 4-6, 6-3, 6-2, 6-4 victory, winning the match with a blistering ace.

Kevin Curren, the South-African born No. 8 seed, who now lives in the U.S., had a tough opening set against Larry Stefanki before winning 7-6 (7-4), 6-3, 6-4.

Gullikson's grass court game gave him the edge against Mecir, the latest tennis star to emerge from Czechoslovakia.

The 20-year-old Mecir has beaten Timmy Connote and Sweden's Mats Wilander this Year and won $15,000 tournaments at Hamburg and Rotterdam, successes that have helped lift him to No. 10 in the world.

He had several chances to beat Gullikson, but let a 3-1 lead slip in the third set and thereafter had to fight for every point.

Play began on the showcourts two hours early to try and make up for the backlog of matches caused by the wet weather on the first three days of the tournament.

But after only 35 minutes of play, the rain returned and play was suspended briefly on all 17 courts.

Mandlikova, the runner-up in 1981, had built a commanding lead by the time her match was held up. She took the opening set in just 18 minutes and led 3-0, 30-40 in the second set.